Compensating for coupling during read operations of non-volatile memory

ABSTRACT

Shifts in the apparent charge stored on a floating gate (or other charge storing element) of a non-volatile memory cell can occur because of the coupling of an electric field based on the charge stored in adjacent floating gates (or other adjacent charge storing elements). The problem occurs most pronouncedly between sets of adjacent memory cells that have been programmed at different times. To compensate for this coupling, the read process for a given memory cell will take into account the programmed state of an adjacent memory cell.

CLAIM OF PRIORITY

This application is a divisional application of U.S. patent applicationSer. No. 11/099,133, “Compensating for Coupling During Read Operationson Non-Volatile Memory,” published as U.S. Publication No.2006-0221692A1, by Chen, et al., filed on Apr. 5, 2005, now U.S. Pat.No. 7,196,928, incorporated herein by reference.

CROSS-REFERENCE TO RELATED APPLICATIONS

This Application is related to U.S. patent application titled“Compensating For Coupling In Non-Volatile Storage,” by Jian Chen,Raul-Adrian Cernea and Gerrit Jan Hemink, U.S. application Ser. No.11/099,239, published as U.S. Publication No. 2006-0221683A1, filed Apr.5, 2005, now U.S. Pat. No. 7,196,946, which is incorporated herein byreference in its entirety.

This Application is related to United States Patent Application titled“Read Operation For Non-Volatile Storage That Includes Compensation ForCoupling,” by Yan Li and Jian Chen, U.S. application Ser. No.11/099,049, published as U.S. Publication No. 2006-0221714A1, filed onApr. 5, 2005, now U.S. Pat. No. 7,187,585, which is incorporated hereinby reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to technology for non-volatile memory.

2. Description of the Related Art

Semiconductor memory has become more popular for use in variouselectronic devices. For example, non-volatile semiconductor memory isused in cellular telephones, digital cameras, personal digitalassistants, mobile computing devices, non-mobile computing devices andother devices. Electrical Erasable Programmable Read Only Memory(EEPROM) and flash memory are among the most popular non-volatilesemiconductor memories.

Both EEPROM and flash memory utilize a floating gate that is positionedabove and insulated from a channel region in a semiconductor substrate.The floating gate is positioned between the source and drain regions. Acontrol gate is provided over and insulated from the floating gate. Thethreshold voltage of the transistor is controlled by the amount ofcharge that is retained on the floating gate. That is, the minimumamount of voltage that must be applied to the control gate before thetransistor is turned on to permit conduction between its source anddrain is controlled by the level of charge on the floating gate.

When programming an EEPROM or flash memory device, such as a NAND flashmemory device, typically a program voltage is applied to the controlgate and the bit line is grounded. Electrons from the channel areinjected into the floating gate. When electrons accumulate in thefloating gate, the floating gate becomes negatively charged and thethreshold voltage of the memory cell is raised so that the memory cellis in a programmed state. More information about programming can befound in U.S. patent application Ser. No. 10/379,608, titled“Self-Boosting Technique,” filed on Mar. 5, 2003; and in U.S. patentapplication Ser. No. 10/629,068, titled “Detecting Over ProgrammedMemory,” filed on Jul. 29, 2003; both applications are incorporatedherein by reference in their entirety.

Some EEPROM and flash memory devices have a floating gate that is usedto store two ranges of charges and, therefore, the memory cell can beprogrammed/erased between two states (an erased state and a programmedstate). Such a flash memory device is sometimes referred to as a binaryflash memory device.

A multi-state flash memory device is implemented by identifying multipledistinct allowed/valid programmed threshold voltage ranges separated byforbidden ranges. Each distinct threshold voltage range corresponds to apredetermined value for the set of data bits encoded in the memorydevice.

Shifts in the apparent charge stored on a floating gate can occurbecause of the coupling of an electric field based on the charge storedin adjacent floating gates. This floating gate to floating gate couplingphenomena is described in U.S. Pat. No. 5,867,429, which is incorporatedherein by reference in its entirety. An adjacent floating gate to atarget floating gate may include neighboring floating gates that are onthe same bit line, neighboring floating gates on the same word line, orfloating gates that are across from the target floating gate becausethey are on both a neighboring bit line and neighboring word line.

The floating gate to floating gate coupling phenomena occurs mostpronouncedly between sets of adjacent memory cells that have beenprogrammed at different times. For example, a first memory cell isprogrammed to add a level of charge to its floating gate thatcorresponds to one set of data. Subsequently, one or more adjacentmemory cells are programmed to add a level of charge to their floatinggates that correspond to a second set of data. After the one or more ofthe adjacent memory cells are programmed, the charge level read from thefirst memory cell appears to be different than programmed because of theeffect of the charge on the adjacent memory cells being coupled to thefirst memory cell. The coupling from adjacent memory cells can shift theapparent charge level being read a sufficient amount to lead to anerroneous reading of the data stored.

The effect of the floating gate to floating gate coupling is of greaterconcern for multi-state devices because in multi-state devices theallowed threshold voltage ranges and the forbidden ranges are narrowerthan in binary devices. Therefore, the floating gate to floating gatecoupling can result in memory cells being shifted from an allowedthreshold voltage range to a forbidden range.

As memory cells continue to shrink in size, the natural programming anderase distributions of threshold voltages are expected to increase dueto short channel effects, greater oxide thickness/coupling ratiovariations and more channel dopant fluctuations, reducing the availableseparation between adjacent states. This effect is much more significantfor multistate memories than memories using only two states (binarymemories). Furthermore, the reduction of the space between word linesand of the space between bit lines will also increase the couplingbetween adjacent floating gates.

Thus, there is a need to reduce the effect of coupling between floatinggates.

SUMMARY OF THE INVENTION

To compensate for the coupling between floating gates, the read processfor a given memory cell will take into account the programmed state ofan adjacent memory cell. Various embodiments are disclosed.

In one embodiment, for each non-volatile storage element of at least asubset of non-volatile storage elements, an offset is determined from apredetermined set of offsets based on a charge level stored in anadjacent storage element. A set of read processes is performed, witheach read process using a different one of the predetermined sets ofoffsets and performed on all of the non-volatile storage elements. Thereis at least one read process for each offset. Each non-volatile storageelement provides final data from the appropriate one of the readprocesses associated with the offset determined for the respectivenon-volatile storage element.

Some embodiments include a method for reading data from a first set ofmulti-state non-volatile storage elements storing data for at least afirst page and a second page. The method includes determining chargelevel data for a second set of multi-state non-volatile storage elementsadjacent to the first set of multi-state non-volatile storage elements.Multiple read processes are performed relative to a first referencevalue for distinguishing threshold voltages relative to two adjacentdata states. Each read process uses a different one of a predeterminedset of offsets with the first reference value. Each of the first set ofnon-volatile storage elements provides final data from the appropriateone of the read processes corresponding to one offset that is associatedwith a respective adjacent non-volatile storage element. Data values aredetermined for the first page based on the final data, without usingother data from user data read processes based on non-zero offsets toreference values between other pairs of adjacent data states.

Some embodiments include programming a first set of non-volatile storageelements and a second set of non-volatile storage elements, both ofwhich store first and second groupings of data. The programming includeswriting to a particular non-volatile storage element with respect to thesecond grouping of data subsequent to writing to adjacent non-volatilestorage elements for the first grouping of data. When intending to readdata from the first set of non-volatile storage elements, the systemwill also read from the second set of non-volatile storage elements. Thereading from the second set of non-volatile storage elements does notrequire reading accurate data, rather the read operation need onlyobtain an indication of a charge level or apparent charge level. Readoperations are performed on the first set of non-volatile storageelements. These read operations use a first compensation fornon-volatile storage elements of the first set of non-volatile storageelements adjacent to non-volatile storage elements of the second set ofnon-volatile storage elements that provide a first indication. Theseread operations do not use the first compensation for non-volatilestorage elements of the first set of non-volatile storage elementsadjacent to non-volatile storage elements of the second set ofnon-volatile storage elements that do not provide the first indication.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top view of a NAND string.

FIG. 2 is an equivalent circuit diagram of the NAND string.

FIG. 3 is a cross-sectional view of the NAND string.

FIG. 4 is a block diagram of an array of NAND flash memory cells.

FIG. 5 is a block diagram of a non-volatile memory system.

FIG. 6 is a block diagram of a non-volatile memory system.

FIG. 7 is a block diagram depicting one embodiment of the sense block.

FIG. 8 is a schematic diagram of one embodiment of a sense module.

FIG. 9 is a table describing a portion of the operation of a sensemodule.

FIG. 10 is a timing diagram describing operation of a sense module.

FIG. 11 is a block diagram explaining a voltage on a common source line.

FIG. 12 depicts a memory cell connected to a source line.

FIG. 13 shows various threshold voltage distributions.

FIG. 14 is a timing diagram explaining a portion of the operation of asense module.

FIG. 15 is a flow chart describing one embodiment of a process forprogramming non-volatile memory.

FIG. 16 is an example wave form applied to the control gates ofnon-volatile memory cells.

FIG. 17 depicts an example set of threshold voltage distributions.

FIG. 18 depicts an example set of threshold voltage distributions.

FIGS. 19A-C show various threshold voltage distributions and describe aprocess for programming non-volatile memory.

FIGS. 20A-G are tables depicting the order of programming non-volatilememory in various embodiments.

FIG. 21 is a flow chart describing one embodiment of a process forreading non-volatile memory.

FIG. 22 is a flow chart describing one embodiment of a process forperforming a read operation for non-volatile memory.

FIG. 23 is a flow chart describing one embodiment of a process forrecovering data.

FIG. 24 is a flow chart describing one embodiment of a process forrecovering data from multiple word lines.

FIG. 25 is a flow chart describing one embodiment of a process forreading data from a lower page.

FIG. 26 is a flow chart describing one embodiment of a process ofreading data from an upper page.

FIG. 27 is a flow chart describing one embodiment of a process forreading data.

FIG. 28 is a flow chart describing one embodiment of a process forreading data from an upper page.

FIG. 29 is a flow chart describing one embodiment of a process forreading data without using compensation.

FIG. 30 is a flow chart describing one embodiment of a process forreading data while compensating for floating gate to floating gate (ordielectric region to dielectric region) coupling.

FIG. 31 is a table depicting a process for determining data values.

FIG. 32 is a flow chart describing one embodiment of a process forreading upper page data using a correction.

FIG. 32A is a timing diagram that describes a pipelined read process.

FIG. 33 is a schematic diagram of a sense module.

FIG. 34 is a flow chart describing one embodiment of a process forreading data.

FIG. 35 is a flow chart describing one embodiment of a process forreading data.

FIG. 36 is a flow chart describing one embodiment of a process forreading upper page data associated with the process of FIG. 35.

DETAILED DESCRIPTION

One example of a memory system suitable for implementing the presentinvention uses the NAND flash memory structure, which includes arrangingmultiple transistors in series between two select gates. The transistorsin series and the select gates are referred to as a NAND string. FIG. 1is a top view showing one NAND string. FIG. 2 is an equivalent circuitthereof. The NAND string depicted in FIGS. 1 and 2 includes fourtransistors, 100, 102, 104 and 106, in series and sandwiched between afirst select gate 120 and a second select gate 122. Select gate 120connects the NAND string to bit line 126. Select gate 122 connects theNAND string to source line 128. Select gate 120 is controlled byapplying the appropriate voltages to control gate 120CG. Select gate 122is controlled by applying the appropriate voltages to control gate122CG. Each of the transistors 100, 102, 104 and 106 has a control gateand a floating gate. Transistor 100 has control gate 100CG and floatinggate 100FG. Transistor 102 includes control gate 102CG and floating gate102FG. Transistor 104 includes control gate 104CG and floating gate104FG. Transistor 106 includes a control gate 106CG and floating gate106FG. Control gate 100CG is connected to word line WL3, control gate102CG is connected to word line WL2, control gate 104CG is connected toword line WL1, and control gate 106CG is connected to word line WL0. Inone embodiment, transistors 100, 102, 104 and 106 are each memory cells.In other embodiments, the memory cells may include multiple transistorsor may be different than that depicted in FIGS. 1 and 2. Select gate 120is connected to select line SGD. Select gate 122 is connected to selectline SGS.

FIG. 3 provides a cross-sectional view of the NAND string describedabove. As depicted in FIG. 3, the transistors of the NAND string areformed in p-well region 140. Each transistor includes a stacked gatestructure that consists of a control gate (100CG, 102CG, 104CG and106CG) and a floating gate (100FG, 102FG, 104FG and 106FG). The floatinggates are formed on the surface of the p-well on top of an oxide orother dielectric film. The control gate is above the floating gate, withan inter-polysilicon dielectric layer separating the control gate andfloating gate. The control gates of the memory cells (100, 102, 104 and106) form the word lines. N+ doped layers 130, 132, 134, 136 and 138 areshared between neighboring cells, whereby the cells are connected to oneanother in series to form a NAND string. These N+ doped layers form thesource and drain of each of the cells. For example, N+ doped layer 130serves as the drain of transistor 122 and the source for transistor 106,N+ doped layer 132 serves as the drain for transistor 106 and the sourcefor transistor 104, N+ doped layer 134 serves as the drain fortransistor 104 and the source for transistor 102, N+ doped layer 136serves as the drain for transistor 102 and the source for transistor100, and N+ doped layer 138 serves as the drain for transistor 100 andthe source for transistor 120. N+ doped layer 126 connects to the bitline for the NAND string, while N+ doped layer 128 connects to a commonsource line for multiple NAND strings.

Note that although FIGS. 1-3 show four memory cells in the NAND string,the use of four transistors is provided only as an example. A NANDstring used with the technology described herein can have less than fourmemory cells or more than four memory cells. For example, some NANDstrings will include 8 memory cells, 16 memory cells, 32 memory cells,etc. The discussion herein is not limited to any particular number ofmemory cells in a NAND string.

Each memory cell can store data represented in analog or digital form.When storing one bit of digital data, the range of possible thresholdvoltages of the memory cell is divided into two ranges, which areassigned logical data “1” and “0.” In one example of a NAND-type flashmemory, the voltage threshold is negative after the memory cell iserased, and defined as logic “1.” The threshold voltage is positiveafter a program operation, and defined as logic “0.” When the thresholdvoltage is negative and a read is attempted by applying 0 volts to thecontrol gate, the memory cell will turn on to indicate logic one isbeing stored. When the threshold voltage is positive and a readoperation is attempted by applying 0 volts to the control gate, thememory cell will not turn on, which indicates that logic zero is stored.

A memory cell can also store multiple states, thereby storing multiplebits of digital data. In the case of storing multiple states of data,the threshold voltage window is divided into the number of states. Forexample, if four states are used, there will be four threshold voltageranges assigned to the data values “11,” “10,” “01” and “00.” In oneexample of a NAND-type memory, the threshold voltage after an eraseoperation is negative and defined as “11.” Positive threshold voltagesare used for the states of “10,” “01,” and “00.” In someimplementations, the data values (e.g., logical states) are assigned tothe threshold ranges using a Gray code assignment so that if thethreshold voltage of a floating gate erroneously shifts to itsneighboring physical state, only one bit will be affected. The specificrelationship between the data programmed into the memory cell and thethreshold voltage ranges of the cell depends upon the data encodingscheme adopted for the memory cells. For example, U.S. Pat. No.6,222,762 and U.S. patent application Ser. No. 10/461,244, “TrackingCells For A Memory System,” filed on Jun. 13, 2003, both of which areincorporated herein by reference in their entirety, describe variousdata encoding schemes for multi-state flash memory cells.

Relevant examples of NAND-type flash memories and their operation areprovided in the following U.S. Patents/Patent Applications, all of whichare incorporated herein by reference in their entirety: U.S. Pat. No.5,570,315; U.S. Pat. No. 5,774,397; U.S. Pat. No. 6,046,935; U.S. Pat.No. 5,386,422; U.S. Pat. No. 6,456,528; and U.S. patent application Ser.No. 09/893,277 (Publication No. US2003/0002348). Other types ofnon-volatile memory in addition to NAND flash memory can also be usedwith the present invention.

Another type of memory cell useful in flash EEPROM systems utilizes anon-conductive dielectric material in place of a conductive floatinggate to store charge in a non-volatile manner. Such a cell is describedin an article by Chan et al., “A True Single-TransistorOxide-Nitride-Oxide EEPROM Device,” IEEE Electron Device Letters, Vol.EDL-8, No. 3, March 1987, pp. 93-95. A triple layer dielectric formed ofsilicon oxide, silicon nitride and silicon oxide (“ONO”) is sandwichedbetween a conductive control gate and a surface of a semi-conductivesubstrate above the memory cell channel. The cell is programmed byinjecting electrons from the cell channel into the nitride, where theyare trapped and stored in a limited region. This stored charge thenchanges the threshold voltage of a portion of the channel of the cell ina manner that is detectable. The cell is erased by injecting hot holesinto the nitride. See also Nozaki et al., “A 1-Mb EEPROM with MONOSMemory Cell for Semiconductor Disk Application,” IEEE Journal ofSolid-State Circuits, Vol. 26, No. 4, April 1991, pp. 497-501, whichdescribes a similar cell in a split-gate configuration where a dopedpolysilicon gate extends over a portion of the memory cell channel toform a separate select transistor. The foregoing two articles areincorporated herein by reference in their entirety. The programmingtechniques mentioned in section 1.2 of “Nonvolatile Semiconductor MemoryTechnology,” edited by William D. Brown and Joe E. Brewer, IEEE Press,1998, incorporated herein by reference, are also described in thatsection to be applicable to dielectric charge-trapping devices. Thememory cells described in this paragraph can also be used with thepresent invention. Thus, the technology described herein also applies tocoupling between dielectric regions of different memory cells.

Another approach to storing two bits in each cell has been described byEitan et al., “NROM: A Novel Localized Trapping, 2-Bit NonvolatileMemory Cell,” IEEE Electron Device Letters, vol. 21, no. 11, November2000, pp. 543-545. An ONO dielectric layer extends across the channelbetween source and drain diffusions. The charge for one data bit islocalized in the dielectric layer adjacent to the drain, and the chargefor the other data bit localized in the dielectric layer adjacent to thesource. Multi-state data storage is obtained by separately readingbinary states of the spatially separated charge storage regions withinthe dielectric. The memory cells described in this paragraph can also beused with the present invention.

FIG. 4 illustrates an example of an array of NAND cells, such as thoseshown in FIGS. 1-3. Along each column, a bit line 206 is coupled to thedrain terminal 126 of the drain select gate for the NAND string 150.Along each row of NAND strings, a source line 204 may connect all thesource terminals 128 of the source select gates of the NAND strings. Anexample of a NAND architecture array and its operation as part of amemory system is found in U.S. Pat. Nos. 5,570,315; 5,774,397; and6,046,935.

The array of memory cells is divided into a large number of blocks ofmemory cells. As is common for flash EEPROM systems, the block is theunit of erase. That is, each block contains the minimum number of memorycells that are erased together. Each block is typically divided into anumber of pages. A page is a unit of programming. In one embodiment, theindividual pages may be divided into segments and the segments maycontain the fewest number of cells that are written at one time as abasic programming operation. One or more pages of data are typicallystored in one row of memory cells. A page can store one or more sectors.A sector includes user data and overhead data. Overhead data typicallyincludes an Error Correction Code (ECC) that has been calculated fromthe user data of the sector. A portion of the controller (describedbelow) calculates the ECC when data is being programmed into the array,and also checks it when data is being read from the array.Alternatively, the ECCs and/or other overhead data are stored indifferent pages, or even different blocks, than the user data to whichthey pertain.

A sector of user data is typically 512 bytes, corresponding to the sizeof a sector in magnetic disk drives. Overhead data is typically anadditional 16-20 bytes. A large number of pages form a block, anywherefrom 8 pages, for example, up to 32, 64 or more pages. In someembodiments, a row of NAND strings comprises a block.

Memory cells are erased in one embodiment by raising the p-well to anerase voltage (e.g., 20 volts) for a sufficient period of time andgrounding the word lines of a selected block while the source and bitlines are floating. Due to capacitive coupling, the unselected wordlines, bit lines, select lines, and c-source are also raised to asignificant fraction of the erase voltage. A strong electric field isthus applied to the tunnel oxide layers of selected memory cells and thedata of the selected memory cells are erased as electrons of thefloating gates are emitted to the substrate side. As electrons aretransferred from the floating gate to the p-well region, the thresholdvoltage of a selected cell is lowered. Erasing can be performed on theentire memory array, separate blocks, or another unit of cells.

FIG. 5 illustrates a memory device 296 having read/write circuits forreading and programming a page of memory cells in parallel, according toone embodiment of the present invention. Memory device 296 may includeone or more memory die 298. Memory die 298 includes a two-dimensionalarray of memory cells 300, control circuitry 310, and read/writecircuits 365. The memory array 300 is addressable by word lines via arow decoder 330 and by bit lines via a column decoder 360. Theread/write circuits 365 include multiple sense blocks 400 and allow apage of memory cells to be read or programmed in parallel. Typically acontroller 350 is included in the same memory device 296 (e.g., aremovable storage card) as the one or more memory die 298. Commands andData are transferred between the host and controller 350 via lines 320and between the controller and the one or more memory die 298 via lines318.

The control circuitry 310 cooperates with the read/write circuits 365 toperform memory operations on the memory array 300. The control circuitry310 includes a state machine 312, an on-chip address decoder 314 and apower control module 316. The state machine 312 provides chip-levelcontrol of memory operations. The on-chip address decoder 314 providesan address interface between that used by the host or a memorycontroller to the hardware address used by the decoders 330 and 360. Thepower control module 316 controls the power and voltages supplied to theword lines and bit lines during memory operations.

FIG. 6 illustrates another arrangement of the memory device 296 shown inFIG. 5. Access to the memory array 300 by the various peripheralcircuits is implemented in a symmetric fashion, on opposite sides of thearray, so that the densities of access lines and circuitry on each sideare reduced by half. Thus, the row decoder is split into row decoders330A and 330B and the column decoder into column decoders 360A and 360B.Similarly, the read/write circuits are split into read/write circuits365A connecting to bit lines from the bottom and read/write circuits365B connecting to bit lines from the top of the array 300. In this way,the density of the read/write modules is essentially reduced by onehalf. The device of FIG. 6 can also include a controller, as describedabove for the device of FIG. 5.

FIG. 7 is a block diagram of an individual sense block 400 partitionedinto a core portion, referred to as a sense module 380, and a commonportion 390. In one embodiment, there will be a separate sense module380 for each bit line and one common portion 390 for a set of multiplesense modules 380. In one example, a sense block will include one commonportion 390 and eight sense modules 380. Each of the sense modules in agroup will communicate with the associated common portion via a data bus372. For further details refer to U.S. patent application Ser. No.11/026,536 “Non-Volatile Memory & Method with Shared Processing for anAggregate of Sense Amplifiers” filed on Dec. 29, 2004 which isincorporated herein by reference in its entirety.

Sense module 380 comprises sense circuitry 370 that determines whether aconduction current in a connected bit line is above or below apredetermined threshold level. Sense module 380 also includes a bit linelatch 382 that is used to set a voltage condition on the connected bitline. For example, a predetermined state latched in bit line latch 382will result in the connected bit line being pulled to a statedesignating program inhibit (e.g., Vdd).

Common portion 390 comprises a processor 392, a set of data latches 394and an I/O Interface 396 coupled between the set of data latches 394 anddata bus 320. Processor 392 performs computations. For example, one ofits functions is to determine the data stored in the sensed memory celland store the determined data in the set of data latches. The set ofdata latches 394 is used to store data bits determined by processor 392during a read operation. It is also used to store data bits importedfrom the data bus 320 during a program operation. The imported data bitsrepresent write data meant to be programmed into the memory. I/Ointerface 398 provides an interface between data latches 394 and thedata bus 320.

During read or sensing, the operation of the system is under the controlof state machine 312 that controls the supply of different control gatevoltages to the addressed cell. As it steps through the variouspredefined control gate voltages corresponding to the various memorystates supported by the memory, the sense module 380 will trip at one ofthese voltages and an output will be provided from sense module 380 toprocessor 392 via bus 372. At that point, processor 392 determines theresultant memory state by consideration of the tripping event(s) of thesense module and the information about the applied control gate voltagefrom the state machine via input lines 393. It then computes a binaryencoding for the memory state and stores the resultant data bits intodata latches 394. In another embodiment of the core portion, bit linelatch 382 serves double duty, both as a latch for latching the output ofthe sense module 380 and also as a bit line latch as described above.

It is anticipated that some implementations will include multipleprocessors 392. In one embodiment, each processor 392 will include anoutput line (not depicted in FIG. 7) such that each of the output linesis wired-OR'd together. In some embodiments, the output lines areinverted prior to being connected to the wired-OR line. Thisconfiguration enables a quick determination during the programverification process of when the programming process has completedbecause the state machine receiving the wired-OR can determine when allbits being programmed have reached the desired level. For example, wheneach bit has reached its desired level, a logic zero for that bit willbe sent to the wired-OR line (or a data one is inverted). When all bitsoutput a data 0 (or a data one inverted), then the state machine knowsto terminate the programming process. Because each processorcommunicates with eight sense modules, the state machine needs to readthe wired-OR line eight times, or logic is added to processor 392 toaccumulate the results of the associated bit lines such that the statemachine need only read the wired-OR line one time. Similarly, bychoosing the logic levels correctly, the global state machine can detectwhen the first bit changes its state and change the algorithmsaccordingly.

During program or verify, the data to be programmed is stored in the setof data latches 394 from the data bus 320. The program operation, underthe control of the state machine, comprises a series of programmingvoltage pulses applied to the control gates of the addressed memorycells. Each programming pulse is followed by a read back (verify) todetermine if the cell has been programmed to the desired memory state.Processor 392 monitors the read back memory state relative to thedesired memory state. When the two are in agreement, the processor 222sets the bit line latch 214 so as to cause the bit line to be pulled toa state designating program inhibit. This inhibits the cell coupled tothe bit line from further programming even if programming pulses appearon its control gate. In other embodiments the processor initially loadsthe bit line latch 382 and the sense circuitry sets it to an inhibitvalue during the verify process.

Data latch stack 394 contains a stack of data latches corresponding tothe sense module. In one embodiment, there are three data latches persense module 380. In some implementations (but not required), the datalatches are implemented as a shift register so that the parallel datastored therein is converted to serial data for data bus 320, and viceversa. In the preferred embodiment, all the data latches correspondingto the read/write block of m memory cells can be linked together to forma block shift register so that a block of data can be input or output byserial transfer. In particular, the bank of r read/write modules isadapted so that each of its set of data latches will shift data in to orout of the data bus in sequence as if they are part of a shift registerfor the entire read/write block.

FIG. 8 illustrates an example of sense module 380; however, otherimplementations can also be used. Sense module 380 comprises a bit lineisolation transistor 512, bit line pull down circuit 520, bit linevoltage clamp 610, readout bus transfer gate 530 and a sense amplifier600 which in this implementation contains bitline latch 382. Note thatthe memory cell 10 and page controller 540 in FIG. 8 are associated withbut not structurally a part of sense module 380.

In general, a page of memory cells is operated on in parallel. Thereforea corresponding number of sense modules are in operation in parallel. Inone embodiment, page controller 540 expediently provides control andtiming signals to the sense modules operated in parallel.

Sense module 380 is connectable to the bit line 36 of a memory cell whenthe bit line isolation transistor 512 is enabled by a signal BLS. Sensemodule 380 senses the conduction current of the memory cell by means ofsense amplifier 600 and latches the read result as a digital voltagelevel SEN2 at a sense node 501 and outputs it to a readout bus 532 viagate 530.

The sense amplifier 600 essentially comprises a second voltage clamp620, a precharge circuit 640, a discriminator or compare circuit 650 anda latch 660. The discriminator circuit 650 includes a dedicatedcapacitor 652. In one embodiment, a reference voltage is applied to thecontrol gate of a memory cell being read. If the reference voltage isgreater than the threshold voltage of the memory cell, then the memorycell will turn on and conduct current between its source and drain. Ifthe reference voltage is not greater than the threshold voltage of thememory cell, then the memory cell will not turn on and will not conductcurrent between its source and drain. In many implementations, theon/off may be a continuous transition so that the memory cell willconduct different currents in response to different control gatevoltages. If the memory cell is on and conducting current, the conductedcurrent will cause the voltage on node SEN 631 to decrease, effectivelycharging or increasing the voltage across capacitor 652 whose otherterminal is at Vdd. If the voltage on node SEN discharges to apredetermined level during a predetermined sensing period, then senseamplifier 600 reports that the memory cell turned on in response to thecontrol gate voltage.

One feature of the sense module 380 is the incorporation of a constantvoltage supply to the bit line during sensing. This is preferablyimplemented by the bit line voltage clamp 610. The bit line voltageclamp 610 operates like a diode clamp with transistor 612 in series withthe bit line 36. Its gate is biased to a constant voltage BLC equal tothe desired bit line voltage VBL above its threshold voltage VT. In thisway, it isolates the bit line from the sense node 501 and sets aconstant voltage level for the bit line, such as the desired VBL=0.5 to0.7 volts during program-verifying or reading. In general, the bit linevoltage level is set to a level such that it is sufficiently low toavoid a long precharge time, yet sufficiently high to avoid ground noiseand other factors.

The sense amplifier 600 senses the conduction current through the sensenode 501 and determines whether the conduction current is above or belowa predetermined value. The sense amplifier outputs the sensed result ina digital form as the signal SEN2 at the sense node 501 to the readoutbus 532.

The digital control signal INV, which is essentially an inverted stateof the signal SEN2 after read, is also output to control the pull downcircuit 520. When the sensed conduction current is higher than thepredetermined value, INV will be HIGH and SEN2 will be LOW. This resultis reinforced by the pull down circuit 520. Pull down circuit 520includes an n-transistor 522 controlled by the control signal INV andanother n-transistor 550 controlled by the control signal GRS. The GRSsignal when LOW allows the bit line 36 to be floated regardless of thestate of the INV signal. During programming, the GRS signal goes HIGH toallow the bit line 36 to be pulled to ground and controlled by INV. Whenthe bit line is required to be floated, the GRS signal goes LOW.

FIGS. 10(H)-10(O) illustrate the timing of the preferred sense moduleshown in FIG. 8. An additional description of the operation of the sensemodule in regards to other features has been described in co-pendingU.S. patent application Ser. No. 10/254,830, “Non-Volatile Memory AndMethod With Reduced Source Line Bias Errors,” filed on Sep. 24, 2002, byRaul-Adrian Cernea and Yan Li, published on Mar. 25, 2004 as Pub. App.No. 2004/0057287; and U.S. patent application Ser. No. 10/665,828,“Non-Volatile Memory And Method with Improved Sensing,” filed on Sep.17, 2003, by Raul-Adrian Cernea and Yan Li, published on Jun. 10, 2004as Pub. App. No. 2004/0109357. The entire disclosure of these tworeferenced applications is hereby incorporated herein by reference intheir entirety.

In one embodiment, a bit line bias is supplied by a bit line voltagecompensator 560. It senses the INV signal from its left and rightneighbors in the form of the signals INVL and INVR, respectively, andresponsively supplies a bias voltage ΔVBL according to the bias voltagetable of FIG. 9. The bias voltage is supplied to node 523 that isswitchably coupled to the bit line 36. During programming, both thesignals BLS and INV are HIGH while the signal GRS is LOW. These enablethe bit line 36 access to the bit line voltage compensator 560.

FIG. 9 is a bias voltage table listing the offset voltage applied to thebit line as a function of the program-inhibit modes of its left andright neighbors. The center column lists the offset or bias voltageapplied to the bit line of the storage unit under programming as afunction of the modes of its left and right neighbors. Generally, themore of its neighbors that are in program-inhibit mode, the more bitline bias is required to offset the perturbation effect of the couplingfrom floating gates of adjacent bit lines.

FIGS. 10(A)-10(G) are timing diagrams illustrating the voltagecompensation scheme during program operations, according to a firstembodiment of the invention.

The voltages shown are applied to various word lines and bit lines ofthe memory array, for NAND strings under programming and programinhibition. The program operation can be grouped into a Bit LinesPrecharge Phase, a Program Phase and a Discharge Phase.

In the Bit Lines Precharge Phase:

(1) The source select transistor is turned off by SGS at 0V (FIG. 10(A))while the drain select transistor is turned on by SGD going high to VSG(FIG. 10(B)), thereby allowing a bit line to access a NAND string.

(2) The bit line voltage of a program-inhibited NAND string is allowedto rise to a predetermined voltage given by VDD (FIG. 10(F)). When thebit line voltage of the program-inhibited NAND string rises to VDD, theprogram-inhibited NAND string will float when the gate voltage SGD onthe drain select transistor drops to VDD. At the same time, the bit linevoltage of a programming NAND string is actively pulled down to 0V (FIG.10(G)).

(3) The bit line voltage of the programming NAND string is biased withΔVBL supplied by the bit line voltage compensator 560 (FIG. 10(G)). Thevalue of ΔVBL output from the voltage compensator 560 depends on whetherone or both of its neighbors is in program-inhibit mode or not.

(4) The drain word line connecting to the drain select transistors of arow of NAND strings has its voltage lowered to VDD. This will float onlythose program-inhibited NAND strings where their bit line voltage iscomparable to VDD, since their drain select transistors are turned off(FIGS. 10(B) & 10(F)). As for the NAND strings containing a memorytransistor to be programmed, their drain select transistors will not beturned off relative to the bit line voltage of near 0V at their drain.

(5) The memory transistors in a NAND string not being addressed havetheir control gate voltage set to VPASS to fully turn them on (FIG.10(C)). Since a program-inhibited NAND string is floating, the highVPASS and Vpgm (program voltage) applied to the control gates of theunaddressed memory transistors boost up the voltages of their channelsand charge storage elements, thereby inhibiting programming. VPASS istypically set to some intermediate voltage (e.g., ˜10V) relative to Vpgm(e.g., ˜15-24V).

In the Program Phase:

(6) Programming voltage Vpgm is applied to the control gate of a memorytransistor selected for programming (FIG. 10(D)). The storage unitsunder program inhibition (i.e., with boosted channels and charge storageunits) will not be programmed. The storage units under programming willbe programmed with a biased bit line voltage (FIG. 10(G)) to offset anyperturbation due to one or both of its neighbors being inprogram-inhibit mode. One perturbation on a programming storage unit isdue to an adjacent storage unit in the word line direction having afloated channel and charge storage unit capacitively boosted by a highcontrol gate voltage from a word line. This occurs when a NAND string isput into program-inhibit mode. This also has the undesirable effect ofperturbing (increasing) the voltage on a charge storage unit of thememory transistor to be programmed. By sensing what its neighbors aredoing during programming of a storage unit, the perturbations of itsneighbors are compensated accordingly with an appropriate bit linevoltage bias.

In the Discharge Phase:

(7) The various control lines and bit lines are allowed to discharge.

One potential issue relevant to sensing memory cells is source linebias. When a large number of memory cells are sensed in parallel, theircombined currents can result in significant voltage rise in a groundloop with finite resistance. This results in a source line bias whichcauses error in a read operation employing threshold voltage sensing.

FIG. 11 illustrates the problem of source voltage error due to currentflow in the source line having a finite resistance to ground. Theread/write circuits 365 operate on a page of memory cellssimultaneously. Each sense module 380 in the read/write circuits 365 iscoupled to a corresponding cell via a bit line. For example, a sensemodule 380 senses the conduction current i₁ (source-drain current) of amemory cell (e.g., cell 1). The conduction current flows from the sensemodule through the bit line into the drain of the memory cell and outfrom the source before going through a source line 204 to ground. In anintegrated circuit chip, the sources of the cells in a memory array areall tied together as multiple branches of the source line 204 connectedto some external ground pad (e.g., Vss pad) of the memory chip. Evenwhen metal strapping is used to reduce the resistance of the sourceline, a finite resistance, R, remains between the source electrode of amemory cell and the ground pad. Typically, the ground loop resistance Ris around 50 ohm.

For the entire page of memory being sensed in parallel, the totalcurrent flowing through the source line 204 is the sum of all theconduction currents, i.e., iTOT =i1+i2+ . . . , +in. Generally, eachmemory cell has a conduction current dependent on the amount of chargeprogrammed into its charge storage element. For a given control gatevoltage of the memory cell, a small charge will yield a comparativelyhigher conduction current. When a finite resistance exists between thesource electrode of a memory cell and the ground pad, the voltage dropacross the resistance is given by Vdrop=iTOT R.

For example, if 4,256 bit lines discharge at the same time, each with acurrent of 1 μA, then the source line voltage drop will be equal to4,000 lines×1 μA/line ×50 ohms ˜0.2 volts. This source line bias willcontribute to a sensing error of 0.2 volts when threshold voltages ofthe memory cells are sensed.

FIG. 12 illustrates the error in the threshold voltage level of a memorycell caused by a source line voltage drop. The threshold voltage VTsupplied to the control gate of the memory cell is relative to GND.However, the effective VT seen by the memory cell is the voltagedifference between its control gate and source. There is a difference ofapproximately Vdrop between the supplied and effective VT (ignoring thesmaller contribution of voltage drop from the source 14 to the sourceline). This Vdrop, or source line bias, will contribute to a sensingerror of, for example, 0.2 volts when threshold voltages of the memorycells are sensed.

According to one aspect of the invention, a method for reducing sourceline bias is accomplished by read/write circuits with features andtechniques for multi-pass sensing. Each pass helps to identify and shutdown the memory cells with conduction current higher than a givendemarcation current value. Typically, with each pass, the givendemarcation current value progressively converges to the breakpointcurrent value for a conventional single-pass sensing. In this way,sensing in subsequent passes will be less affected by source line biassince the higher current cells have been shut down.

FIG. 13 illustrates an example population distribution of a page ofmemory cells for a 4-state memory. Each cluster of memory cells isprogrammed within a range of conduction currents ISD clearly separatedfrom each other. For example, a breakpoint 381 is a demarcating currentvalue between two clusters, respectively representing the “A” and “B”memory states. In a conventional single-pass sensing, a necessarycondition for a “B” memory state will be that it has a conductioncurrent less than the breakpoint 381. If there were no source line bias,the population distribution with respect to the supplied thresholdvoltage VT would be depicted by the curve with the solid line. However,because of the source line bias error, the threshold voltage of each ofthe memory cells at its control gate is increased by the source linebias. This means a higher control gate voltage need be applied tocompensate for the bias. In FIG. 13, the source line bias results in ashifting of the distribution (broken line) towards a higher apparent VT.The shifting will be more when sensing the higher threshold (lowercurrent) memory states since more total array current is flowing due tothe higher applied word line voltage. If the breakpoint 381 is designedfor the case without source line error, then the existence of a sourceline error will have some of the tail end of “A” states havingconduction currents appear in a region of no conduction, which means itwould be higher than the breakpoint 381. This will result in some of the“A” states (more conducting) being mistakenly demarcated as “B” states(less conducting).

For example, the present multi-pass sensing can be implemented in twopasses (j=1 to 2). After the first pass, those memory cells withconduction currents higher than the breakpoint 381 are identified andremoved by turning off their conduction current. A preferred way to turnoff their conduction currents is to set their drain voltages on theirbit lines to ground. In a second pass (j=2), because of the removal ofthe high current states that contributed to the source line bias, thedistribution with the broken line approaches that of the one with thesolid line. Thus, sensing using the breakpoint 381 as the demarcationcurrent value will not result in mistaking the “A” states for the “B”states.

As compared to a conventional one-pass approach, the present two-passmethod substantially reduces the likelihood of misidentifying some ofthe “A” cells as “B” or higher cells. More than two passes are alsocontemplated, although there will be diminishing returns with increasingnumber of passes. Further, each pass may have the same demarcationcurrent, or with each successive pass, the demarcation current usedconverges to that of a breakpoint normally used in a conventionalsingle-pass sensing. Additionally, breakpoints can be used betweenstates E and A, as well as between states B and C.

In general, there will be a page of memory cells being operated on by acorresponding number of multi-pass sense blocks 400. A page controller540 supplies control and timing signals to each of the sense modules. Inone embodiment, the page controller 540 is implemented as part of thestate machine 312 in the control circuitry 310. In another embodiment,page controller 540 is part of the read/write circuits 365. Pagecontroller 540 cycles each of the multi-pass sense blocks 400 through apredetermined number of passes (j=1 to N) and also supplies apredetermined demarcation current value I₀(j) for each pass. Thedemarcation current value can also be implemented as a time period forsensing. After the last pass, the page controller 540 enables transfergate 488 with a signal NCO to read the state of the SEN node 631 assensed data to a readout bus 532. In all, a page of sense data will beread out from all of the sense modules.

Additional operation and timing of the sense module 380 duringread/verify operations will be discussed with respect to the timingdiagrams FIGS. 14(A)-14(K), which are demarcated into PHASES (1)-(9).

PHASE (0): Setup

The sense module 380 (see FIG. 8) is connected to the bit line 36 via anenabling signal BLS (FIG. 14(A)). The Voltage clamp is enabled with BLC(FIG. 14(B)). The Precharge circuit 640 is enabled as a limited-currentsource with a control signal FLT (FIG. 14(C)).

PHASE (1): Controlled Precharge

Sense amplifier 600 is initialized by a reset signal RST (FIG. 14(D))which will pull the signal INV to ground via the transistor 658. Thus,on reset, INV is set to LOW. At the same time, a p-transistor 663 pullsa complementary signal LAT to V_(dd) or HIGH (FIG. 14(H)).

Isolation gate 630 is formed by an n-transistor 632, which is controlledby the signal LAT. Thus, after reset, the isolation gate is enabled toconnect sense node 501 to the sense amplifier's internal sense node 631,and the signal SEN2 will be the same as the signal SEN at the internalsense node 631.

The precharge circuit 640 precharges the bit line 36 through theinternal sense node 631 and the sense node SEN2 501 for a predeterminedperiod of time. This will bring the bit line to an optimal voltage forsensing the conduction therein.

The precharge circuit 640 includes a pull-up p-transistor 642 controlledby the control signal FLT (“FLOAT”). The bit line will be pulled uptowards the desired bit line voltage as set by the bit line voltageclamp 610. The rate of pull-up will depend on the conduction current inthe bit line. The smaller the conduction current, the faster thepull-up.

It has been described earlier that sensing errors due to the source linebias are minimized if those memory cells with conduction currents higherthan a predetermined value are turned off and their contributions to thesource line bias eliminated. Precharge circuit 640 is implemented toserve two functions. One is to precharge the bit line to an optimalsensing voltage. The other is to help identify those memory cells withconduction currents higher than a predetermined value for D.C. (DirectCurrent) sensing so that they may be eliminated from contributing tosource line bias.

The D.C. sensing is accomplished by providing a precharge circuit thatbehaves like a current source for supplying a predetermined current tothe bit line. The signal FLT that controls the p-transistor 642 is suchthat it “programs” a predetermined current to flow through the prechargecircuit 640. As an example, the FLT signal may be generated from acurrent mirror with a reference current set to 500 nA. When thep-transistor 642 forms the mirrored leg of the current mirror, it willalso have the same 500 nA throwing in it.

FIGS. 14(I1)-14(I4) illustrate the voltages on four example bit linesconnected respectively to memory cells with conduction currents of 700nA, 400 nA, 220 nA and 40 nA. When the precharge circuit 640 is acurrent source with a limit of 500 nA, for example, a memory cell havinga conduction current exceeding 500 nA will have the charges on the bitline drained faster than it can accumulate. Consequently, for the bitline with conduction current 700 nA, its voltage or the signal SEN atthe internal sense node 631 will remain close to 0V (such as 0.1 volt;see FIG. 14(I1)). On the other hand, if the memory cell's conductioncurrent is below 500 nA, the precharge circuit 640 will begin to chargeup the bit line and its voltage will begin to rise towards the clampedbit line voltage (e.g., 0.5V set by the voltage clamp 610) (FIGS.14(I2)-14(I4)). Correspondingly, the internal sense node 631 will eitherremain close to 0V or be pulled up to Vdd (FIG. 14(G)). Generally, thesmaller the conduction current, the faster the bit line voltage willcharge up to the clamped bit line voltage. Thus, by examining thevoltage on a bit line after the controlled precharge phase, it ispossible to identify if the connected memory cell has a conductioncurrent higher or lower than a predetermined level.

PHASE (2): D.C. Latching & Removing the High Current Cells fromSubsequent Strobes

After the controlled precharge phase, an initial D.C. high-currentsensing phase begins where the signal SEN is sensed by the discriminatorcircuit 650. The sensing identifies those memory cells with conductioncurrents higher than the predetermined level. The discriminator circuit650 includes two p-transistors 654 and 656 in series, which serve as apull up for a node 657 registering the signal INV. The p-transistor 654is enabled by a read strobe signal STB going LOW and the p-transistor656 is enabled by the SEN signal at the internal sense node 631 goingLOW. High current memory cells will have the signal SEN close to 0V orat least unable for its bit lines to be precharged sufficiently high toturn off the p-transistor 656. For example, if the weak pull up islimited to a current of 500 nA, it will fail to pull up a cell withconduction current of 700 nA (FIG. 14(G1)). When STB strobes LOW tolatch, INV at the node 657 is pulled up to Vdd. This will set the latchcircuit 660 with INV HIGH and LAT LOW (FIG. 14(H1)).

When INV is HIGH and LAT LOW, the isolation gate 630 is disabled and thesense node 481 is blocked from the internal sense node 631. At the sametime, the bit line is pulled to ground by the pull down circuit 520(FIGS. 8 & 14(I1)). This will effectively turn off any conductioncurrent in the bit line, eliminating it from contributing to source linebias.

Thus, in one preferred implementation of the sense module 380, alimited-current source precharge circuit is employed. This provides anadditional or alternative way (D.C. sensing) to identify bit linescarrying high currents and to turn them off to minimize source line biaserror in subsequent sensing.

In another embodiment, the precharge circuit is not specificallyconfigured to help identify high current bit lines but is optimized topull up and precharge the bit line as fast as possible within theallowance of the maximum current available to the memory system.

PHASE (3): Recovery/Precharge

Prior to a sensing of the conduction current in a bit line that has notbeen previously pulled down, the precharge circuit is activated by thesignal FLT going LOW to precharge the internal sense node 631 to V_(dd)(FIG. 14(C) and FIGS. 14(I2)-14(I4)) and the bit line which may havebeen partially coupled down due to a decrease in the voltage on adjacentbit lines.

PHASE (4): First A.C. Sensing

In one embodiment, an A.C. (Alternating Current or transient) sensing isperformed by determining the voltage drop at the floated internal sensenode 631. This is accomplished by the discriminator or compare circuit650 employing the capacitor C_(SA) 652 coupled to the internal sensenode 631, and considering the rate the conduction current is charging it(reducing the voltage on node SEN). In an integrated circuitenvironment, the capacitor 652 is typically implemented with atransistor; however, other implementations are suitable. Capacitor 652has a predetermined capacitance, e.g., 30 fF, which can be selected foroptimal current determination. The demarcation current value, typicallyin the range of 100-1000 nA, can be set by appropriate adjustment of thecharging period.

The discriminator circuit 650 senses the signal SEN in the internalsense node 631. Prior to each sensing, the signal SEN at the internalsense node 631 is pulled up to V_(dd) by the precharge circuit 640. Thiswill initially set the voltage across the capacitor 652 to be zero.

When the sense amplifier 600 is ready to sense, the precharge circuit640 is disabled by FLT going HIGH (FIG. 14(C)). The first sensing periodT1 is ended by the assertion of the strobe signal STB. During thesensing period, a conduction current induced by a conducting memory cellwill charge the capacitor. The voltage at SEN will decrease from V_(dd)as the capacitor 652 is charged through the draining action of theconduction current in the bit line. FIG. 14(G) (see curves G2-G4)illustrates the SEN signal corresponding to the remaining three examplebit lines connected respectively to memory cells with conductioncurrents of 400 nA, 220 nA and 40 nA, the decrease being more rapid forthose with a higher conduction current.

PHASE (5): First A.C. Latching and Removal of Higher Current Cells fromSubsequent Sensing

At the end of the first predetermined sensing period, SEN will havedecreased to some voltage depending on the conduction current in the bitline (see curves G2-G4 of FIG. 14G). As an example, the demarcationcurrent in this first phase is set to be at 300 nA. The capacitor C_(SA)652, the sensing period T1 and the threshold voltage of the p-transistor656 are such that the signal SEN corresponding to a conduction currenthigher than the demarcation current (e.g., 300 nA) will dropsufficiently low to turn on the transistor 656 in the discriminatorcircuit 650. When latching signal STB strobes LOW, the output signal INVwill be pulled HIGH, and will be latched by the latch 660 (FIG. 14(E)and FIG. 14(H) (curve H2)). On the other hand, the signal SENcorresponding to a conduction current below the demarcation current willproduce a signal SEN unable to turn on the transistor 656. In this case,the latch 660 will remain unchanged, in which case LAT remains HIGH(FIGS. 14(H3) and 14(H4)). Thus, it can be seen that the discriminatorcircuit 650 effectively determines the magnitude of the conductioncurrent in the bit line relative to a reference current set by thesensing period.

Sense amplifier 600 also includes the second voltage clamp 620 whosepurpose is to maintain the voltage of the drain of the transistor 612sufficiently high in order for the bit line voltage clamp 610 tofunction properly. As described earlier, the bit line voltage clamp 610clamps the bit line voltage to a predetermined value VBL, e.g., 0.5V.This will require the gate voltage BLC of the transistor 612 to be setat VBL+VT (where VT is the threshold voltage of the transistor 612) andthe drain connected to the sense node 501 to be greater than the source,i.e., the signal SEN2>VBL. In particular, given the configurations ofthe voltage clamps 610 and 620, SEN2 should be no higher than thesmaller of LAT−VT or BLX−VT, and SEN should be no lower. During sensing,the isolation gate 630 is in a pass-through mode. However, duringsensing the signal SEN at the internal sense node 631 has a voltage thatdecreases from V_(dd). The second voltage clamp 620 prevents SEN fromdropping below LAT−VT or BLX−VT, whichever is lower. This isaccomplished by an n-transistor 612 controlled by a signal BLX, whereBLX is ≧VBL+VT (FIG. 13(F)). Thus, through the actions of the voltageclamps 610 and 620, the bit line voltage VBL is kept constant, e.g.,˜0.5V, during sensing.

Measuring current using a dedicated capacitor 652 instead of traditionaluse of the bit line capacitance is advantageous in several respects.First, it allows a constant voltage source on the bit line therebyavoiding bit-line to bit-line crosstalk. Secondly, the dedicatedcapacitor 652 allows a capacitance to be selected that is optimal forsensing. For example, it may have a capacitance of about 30 fF ascompared to a bit line capacitance of about 2 pF. A smaller capacitancecan increase the sensing speed since it charges faster. Finally, sensingrelative to a dedicated capacitance as compared to the prior art methodof using the capacitance of the bit line allows the sensing circuits tobe independent of the memory architecture or size.

In another embodiment, the current determination is accomplished bycomparison with a reference current, which may be provided by theconduction current of a reference memory cell. This could be implementedwith the compare current as part of a current mirror.

The output of the current determination is latched by the latch circuit660. The latch circuit is formed as a Set/Reset latch by the transistors661, 662, 663 and 664 together with the transistors 666 and 668. Thep-transistor 666 is controlled by the signal RST (RESET) and then-transistor 668 is controlled by the signal STB. A variation of theabove-described sense amplifier that is adapted for low voltageoperation is found in U.S. patent application Ser. No. 11/015,199 titled“Improved Memory Sensing Circuit And Method For Low Voltage Operation,”Inventor Raul-Adrian Cernea, filed on Dec. 16, 2004, incorporated hereinby reference in its entirety.

In general, there will be a page of memory cells being operated on by acorresponding number of multi-pass sense modules 380 (FIG. 8). For thosememory cells having conduction current higher than the first demarcationcurrent level, their LAT signal will be latched LOW (INV latched HIGH).This in turn activates the bit line pull down circuit 520 to pull thecorresponding bit lines to ground, thereby turning off their currents.

PHASE (6): Recovery/Precharge

Prior to the next sensing of the conduction current in a bit line suchas bit line 36 that has not been previously pulled down, the prechargecircuit is activated by the signal FLT to precharge the internal sensenode 631 to V_(dd) (FIG. 14(C)(6) and FIGS. 14(I3)(6)-14(I4)(6)).

PHASE (7): Second Sensing

When the sense amplifier 600 is ready to sense, the precharge circuit642 is disabled by FLT going HIGH (FIG. 14(C)). The second sensingperiod T2 is set by the assertion of the strobe signal STB. During thesensing period, a conduction current, if any, will charge the capacitor.SEN will decrease from V_(dd) as the capacitor 652 is charging throughthe draining action of the conduction current in the bit line 36.

In accordance with the example before, the memory cells with conductioncurrents higher than 300 nA have already been identified and shut downin the earlier phases. FIG. 14(G) (curves G3 and G4) illustraterespectively the SEN signal corresponding to the two example bit linesconnected respectively to memory cells with conduction currents of 220nA and 40 nA.

PHASE (8): Second Latching for Reading Out

At the end of the second predetermined sensing period T2, SEN will havedecreased to some voltage depending on the conduction current in the bitline 36 (FIG. 14(G) (curves G3 and G4)). As an example, the demarcationcurrent in this second phase is set to be at 100 nA. In this case, thememory cell with the conduction current 220 nA will have its INV latchedHIGH (FIG. 14(H)) and its bit line subsequently pulled to ground (FIG.14(I3)). On the other hand, the memory cell with the conduction current40 nA will have no effect on the state of the latch, which was presetwith LAT HIGH.

PHASE (9): Read Out to the Bus

Finally, in the read out phase, the control signal NCO at the transfergate 488 allows the latched signal SEN2 to be read out to the readoutbus 499 (FIGS. 14(J) and 14(K)).

As can be seen from FIGS. 14(I1)-14(I4), the bit line voltage remainsconstant during each sensing period. Thus, from the discussion earlier,capacitive bit-line to bit-line coupling is eliminated.

The sense module 380 described above is one embodiment where sensing isperformed with three passes, the first two passes being implemented toidentify and shut down higher current memory cells. With the highercurrent contributions to the source line bias eliminated, the final passis able to sense the cells with lower range conduction currents moreaccurately.

In other embodiments, sensing operations are implemented with differentcombination of D.C. and A.C. passes, some using only two or more A.C.passes, or only one pass. For the different passes, the demarcationcurrent value used may be the same each time or converge progressivelytowards the demarcation current used in the final pass.

FIG. 15 is a flow chart describing one embodiment of a method forprogramming non-volatile memory. In one implementation, memory cells areerased (in blocks or other units) prior to programming. In step 700 ofFIG. 15, a “data load” command is issued by the controller and inputreceived by control circuitry 310. In step 702, address data designatingthe page address is input to decoder 314 from the controller or host. Instep 704, a page of program data for the addressed page is input to adata buffer for programming. That data is latched in the appropriate setof latches. In step 706, a “program” command is issued by the controllerto state machine 312.

Triggered by the “program” command, the data latched in step 704 will beprogrammed into the selected memory cells controlled by state machine312 using the stepped pulses of FIG. 16 applied to the appropriate wordline. In step 708, the program voltage Vpgm is initialized to thestarting pulse (e.g., 12V) and a program counter PC maintained by statemachine 312 is initialized at 0. In step 710, the first Vpgm pulse isapplied to the selected word line. If logic “0” is stored in aparticular data latch indicating that the corresponding memory cellshould be programmed, then the corresponding bit line is grounded. Onthe other hand, if logic “1” is stored in the particular latchindicating that the corresponding memory cell should remain in itscurrent data state, then the corresponding bit line is connected to Vddto inhibit programming.

In step 712, the states of the selected memory cells are verified. If itis detected that the target threshold voltage of a selected cell hasreached the appropriate level, then the data stored in the correspondingdata latch is changed to a logic “1.” If it is detected that thethreshold voltage has not reached the appropriate level, the data storedin the corresponding data latch is not changed. In this manner, a bitline having a logic “1” stored in its corresponding data latch does notneed to be programmed. When all of the data latches are storing logic“1,” the state machine (via the wired-OR type mechanism described above)knows that all selected cells have been programmed. In step 714, it ischecked whether all of the data latches are storing logic “1.” If so,the programming process is complete and successful because all selectedmemory cells were programmed and verified. A status of “PASS” isreported in step 716.

If, in step 714, it is determined that not all of the data latches arestoring logic “1,” then the programming process continues. In step 718,the program counter PC is checked against a program limit value PCMAX.One example of a program limit value is 20; however, other numbers canalso be used. If the program counter PC is not less than 20, then theprogram process has failed and a status of “FAIL” is reported in step720. If the program counter PC is less than 20, then the Vpgm level isincreased by the step size and the program counter PC is incremented instep 722. After step 722, the process loops back to step 710 to applythe next Vpgm pulse.

At the end of a successful program process, the threshold voltages ofthe memory cells should be within one or more distributions of thresholdvoltages for programmed memory cells or within a distribution ofthreshold voltages for erased memory cells, as appropriate. FIG. 17illustrates threshold voltage distributions for the memory cell arraywhen each memory cell stores two bits of data. FIG. 17 shows a firstthreshold voltage distribution E for erased memory cells. Threethreshold voltage distributions, A, B and C for programmed memory cells,are also depicted. In one embodiment, the threshold voltages in the Edistribution are negative and the threshold voltages in the A, B and Cdistributions are positive.

Each distinct threshold voltage range of FIG. 17 corresponds topredetermined values for the set of data bits. The specific relationshipbetween the data programmed into the memory cell and the thresholdvoltage levels of the cell depends upon the data encoding scheme adoptedfor the cells. For example, U.S. Pat. No. 6,222,762 and U.S. patentapplication Ser. No. 10/461,244, “Tracking Cells For A Memory System,”filed on Jun. 13, 2003, both of which are incorporated herein byreference in their entirety, describe various data encoding schemes formulti-state flash memory cells. In one embodiment, data values areassigned to the threshold voltage ranges using a Gray code assignment sothat if the threshold voltage of a floating gate erroneously shifts toits neighboring physical state, only one bit will be affected. Oneexample assigns “11” to threshold voltage range E (state E), “10” tothreshold voltage range A (state A), “00” to threshold voltage range B(state B) and “01” to threshold voltage range C (state C). However, inother embodiments, Gray code is not used. Although FIG. 17 shows fourstates, the present invention can also be used with other multi-statestructures including those that include more or less than four states.

FIG. 17 also shows three read reference voltages, Vra, Vrb and Vrc, forreading data from memory cells. By testing whether the threshold voltageof a given memory cell is above or below Vra, Vrb and Vrc, the systemcan determine what state the memory cell is in. FIG. 17 also shows threeverify reference voltages, Vva, Vvb and Vvc. When programming memorycells to state A, the system will test whether those memory cells have athreshold voltage greater than or equal to Vva. When programming memorycells to state B, the system will test whether the memory cells havethreshold voltages greater than or equal to Vvb. When programming memorycells to state C, the system will determine whether memory cells havetheir threshold voltage greater than or equal to Vvc.

In one embodiment, known as full sequence programming, memory cells canbe programmed from the erase state E directly to any of the programmedstates A, B or C. For example, a population of memory cells to beprogrammed may first be erased so that all memory cells in thepopulation are in erased state E. The process depicted in FIG. 15, usingthe control gate voltage sequence depicted in FIG. 16, will then be usedto program memory cells directly into states A, B or C. While somememory cells are being programmed from state E to state A, other memorycells are being programmed from state E to state B and/or from state Eto state C. When programming from state E to state C on WLn, the amountof coupling to the adjacent floating gate under WLn−1 is a maximum sincethe change of voltage on the floating gate under WLn is large. Whenprogramming from state E to state B the amount of coupling to theadjacent floating gate is reduced but still significant. Whenprogramming from state E to state A the amount of coupling is reducedeven further. Consequently the amount of correction required tosubsequently read each state of WLn−1 will vary depending on the stateof the adjacent cell on WLn.

FIG. 18 illustrates an example of a two-pass technique of programming amulti-state memory cell that stores data for two different pages: alower page and an upper page. Four states are depicted: state E (11),state A (10), state B (00) and state C (01). For state E, both pagesstore a “1.” For state A, the lower page stores a “0” and the upper pagestores a “1.” For state B, both pages store “0.” For state C, the lowerpage stores “1” and the upper page stores “0.” Note that althoughspecific bit patterns have been assigned to each of the states,different bit patterns may also be assigned. In a first programmingpass, the cell's threshold voltage level is set according to the bit tobe programmed into the lower logical page. If that bit is a logic “1,”the threshold voltage is not changed since it is in the appropriatestate as a result of having been earlier erased. However, if the bit tobe programmed is a logic “0,” the threshold level of the cell isincreased to be state A, as shown by arrow 730. That concludes the firstprogramming pass.

In a second programming pass, the cell's threshold voltage level is setaccording to the bit being programmed into the upper logical page. Ifthe upper logical page bit is to store a logic “1,” then no programmingoccurs since the cell is in one of the states E or A, depending upon theprogramming of the lower page bit, both of which carry an upper page bitof “1.” If the upper page bit is to be a logic “0,” then the thresholdvoltage is shifted. If the first pass resulted in the cell remaining inthe erased state E, then in the second phase the cell is programmed sothat the threshold voltage is increased to be within state C, asdepicted by arrow 734. If the cell had been programmed into state A as aresult of the first programming pass, then the memory cell is furtherprogrammed in the second pass so that the threshold voltage is increasedto be within state B, as depicted by arrow 732. The result of the secondpass is to program the cell into the state designated to store a logic“0” for the upper page without changing the data for the lower page. Inboth FIG. 17 and FIG. 18 the amount of coupling to the floating gateunder the adjacent word line depends on the final state.

In one embodiment, a system can be set up to perform full sequencewriting if enough data is written to fill up an entire page. If notenough data is written for a full page, then the programming process canprogram the lower page programming with the data received. Whensubsequent data is received, the system will then program the upperpage. In yet another embodiment, the system can start writing in themode that programs the lower page and convert to full sequenceprogramming mode if enough data is subsequently received to fill up anentire (or most of a) word line's memory cells. More details of such anembodiment are disclosed in U.S. Patent Application titled “PipelinedProgramming of Non-Volatile Memories Using Early Data,” Ser. No.11/013,125, filed on Dec. 14, 2004, inventors Sergy AnatolievichGorobets and Yan Li, incorporated herein by reference in its entirety.

FIGS. 19A-C disclose another process for programming non-volatile memorythat reduces floating gate to floating gate coupling by, for anyparticular memory cell, writing to that particular memory cell withrespect to a particular page subsequent to writing to adjacent memorycells for previous pages. In one example of an implementation of theprocess taught by FIGS. 19A-C, the non-volatile memory cells store twobits of data per memory cell, using four data states. For example,assume that state E is the erased state and states A, B and C are theprogrammed states. State E stores data 11. State A stores data 01. StateB stores data 10. State C stores data 00. This is an example of non-Graycoding because both bits change between adjacent states A & B. Otherencodings of data to physical data states can also be used. Each memorycell stores two pages of data. For reference purposes these pages ofdata will be called upper page and lower page; however, they can begiven other labels. With reference to state A for the process of FIG.19, the upper page stores bit 0 and the lower page stores bit 1. Withreference to state B, the upper page stores bit 1 and the lower pagestores bit 0. With reference to state C, both pages store bit data 0.The programming process of FIG. 19 is a two-step process. In the firststep, the lower page is programmed. If the lower page is to remain data1, then the memory cell state remains at state E. If the data is to beprogrammed to 0, then the threshold of voltage of the memory cell israised such that the memory cell is programmed to state B′. FIG. 19Atherefore shows the programming of memory cells from state E to stateB′. State B′ depicted in FIG. 19A is an interim state B; therefore, theverify point is depicted as Vvb′, which is lower than Vvb.

In one embodiment, after a memory cell is programmed from state E tostate B′, its neighbor memory cell (WLn+1) in the NAND string will thenbe programmed with respect to its lower page. For example, looking backat FIG. 2, after the lower page for memory cell 106 is programmed, thelower page for memory cell 104 would be programmed. After programmingmemory cell 104, the floating gate to floating gate coupling effect willraise the apparent threshold voltage of memory cell 106 if memory cell104 had a threshold voltage raised from state E to state B′. This willhave the effect of widening the threshold voltage distribution for stateB′ to that depicted as threshold voltage distribution 750 of FIG. 19B.This apparent widening of the threshold voltage distribution will beremedied when programming the upper page.

FIG. 19C depicts the process of programming the upper page. If thememory cell is in erased state E and the upper page is to remain at 1,then the memory cell will remain in state E. If the memory cell is instate E and its upper page data is to be programmed to 0, then thethreshold voltage of the memory cell will be raised so that the memorycell is in state A. If the memory cell was in intermediate thresholdvoltage distribution 750 and the upper page data is to remain at 1, thenthe memory cell will be programmed to final state B. If the memory cellis in intermediate threshold voltage distribution 750 and the upper pagedata is to become data 0, then the threshold voltage of the memory cellwill be raised so that the memory cell is in state C. The processdepicted by FIGS. 19A-C reduces the effect of floating gate to floatinggate coupling because only the upper page programming of neighbor memorycells will have an effect on the apparent threshold voltage of a givenmemory cell. An example of an alternate state coding is to move fromdistribution 750 to state C when the upper page data is a 1, and to moveto state B when the upper page data is a 0.

Although FIGS. 19A-C provide an example with respect to four data statesand two pages of data, the concepts taught by FIGS. 19A-C can be appliedto other implementations with more or less than four states anddifferent than two pages.

FIGS. 20A-F depict various tables that describe the order of programmingaccording to various embodiments for the methods described by FIGS. 17,18 and 19. As described above, each block includes a set of bit linesforming columns and a set of word lines forming rows. In one embodiment,the bit lines are divided into odd bit lines and even bit lines. Memorycells along a common word line and connected to the odd bit lines areprogrammed at one time, while memory cells along a common word line andconnected to even bit lines are programmed at another time (“odd/evenprogramming”). In another embodiment, memory cells are programmed alonga word line for all bit lines in the block (“all bit line programming”).In other embodiments, the bit lines or block can be broken up into othergroupings (e.g., left and right, more than two groupings, etc.).

FIG. 20A is a table which describes the order for programming memorycells along a bit line for all bit line programming. In this embodiment,the block with four word lines includes four pages (page 0-3). Page 0 iswritten first, followed by page 1, followed by page 2 and then followedby page 3. The data in page 0 includes the data stored by all the memorycells connected to word line WL0. The data in page 1 includes the datastored by the memory cells connected to word line WL1. The data in page2 includes the data stored by memory cells connected to WL2. The data inpage 3 includes the data stored by memory cells connected to word lineWL3. The embodiment of FIG. 20A assumes full sequence programming, asdescribed above with respect to FIG. 17.

In another embodiment of full sequence programming, the data can bewritten first to even bit lines and then to odd bit lines. FIG. 20Bdepicts the order of programming even and odd bit lines when using thefull sequence programming method described above with respect to FIG.17. In this embodiment, a block with four word lines includes eightpages of data. The memory cells on even bit lines connected to word lineWL0 store data for page 0. Memory cells on odd bit lines connected toword line WL0 store data for page 1. Memory cells on even bit linesconnected to word line WL1 store data for page 2. Memory cells on oddbit lines connected to word line WL1 store data for page 3. Memory cellson even bit lines connected to word line WL2 store data for page 4.Memory cells on odd bit lines connected to word line WL2 store data forpage 5. Memory cells on even bit lines connected to word line WL3 storedata for page 6. Memory cells on odd bit lines connected to word lineWL3 store data for page 7. Data is programmed in numerical orderaccording to page number, from page 0 to page 7.

The table of FIG. 20C describes the order for programming according tothe two phase programming process of FIG. 18 for a memory array thatperforms all bit line programming. A block with four word lines isdepicted to include eight pages. For memory cells connected to word lineWL0, the lower page of data forms page 0 and the upper page data formspage 1. For memory cells connected to word line WL1, the lower page ofdata forms page 2 and the upper page data forms page 3. For memory cellsconnected to word line WL2, the lower page of data forms page 4 and theupper page data forms page 5. For memory cells connected to word lineWL3, the lower page of data forms page 6 and the upper page data formspage 7. Data is programmed in numerical order according to page number,from page 0 to page 7.

FIG. 20D provides a table describing the order of programming thetwo-phase programming process of FIG. 18 for a memory architecture thatperforms odd/even programming. A block with four word lines includes 16pages, where the pages are programmed in numerical order according topage number, from page 0 to page 15. For memory cells on even bit linesconnected to word line WL0, the lower page of data forms page 0 and theupper page data forms page 2. For memory cells on odd bit linesconnected to word line WL0, the lower page of data forms page 1 and theupper page of data forms page 3. For memory cells on even bit linesconnected to word line WL1, the lower page forms page 4 and the upperpage forms page 6. For memory cells on odd bit lines connected to wordline WL1, the lower page forms page 5 and the upper page forms page 7.For memory cells on even bit lines connected to word line WL2, the lowerpage forms page 8 and the upper page forms page 10. For memory cells onodd bit lines connected to word line WL2, the lower page forms page 9and the upper page forms page 11. For memory cells on even bit linesconnected to word line WL3, the lower page forms page 12 and the upperpage forms page 14. For memory cells on odd bit lines connected to wordline WL3, the lower page forms page 13 and the upper page forms page 15.Alternately, as in FIG. 20E, both lower and upper pages under eachwordline of the even bitlines are programmed before programming bothpages of the odd bit lines for this same word line.

FIGS. 20F and 20G describe the order for programming memory cellsutilizing the programming method of FIGS. 19A-C. FIG. 20F pertains tothe architecture that performs all bit line programming. For memorycells connected to word line WL0, the lower page forms page 0 and theupper page forms page 2. For memory cells connected to word line WL1,the lower page forms page 1 and the upper page forms page 4. For memorycells connected to word line WL2, the lower page forms page 3 and theupper page forms page 6. For memory cells connected to word line WL3,the lower page forms page 5 and the upper page forms page 7. Memorycells are programmed in numerical order according to page number, frompage 0 to page 7.

The table of FIG. 20G pertains to the architecture that performsodd/even programming. For memory cells on even bit lines connected toword line WL0, the lower page forms page 0 and the upper page forms page4. For memory cells on odd bit lines connected to word line WL0, thelower page forms page 1 and the upper page forms page 5. For memorycells on even bit lines connected to word line WL1, the lower page formspage 2 and the upper page forms page 8. For the memory cells on odd bitlines connected to word line WL1, the lower page forms page 3 and theupper page forms page 9. For the memory cells on even bit linesconnected to word line WL2, the lower page forms page 6 and the upperpage forms page 12. For the memory cells on odd bit lines connected toword line WL2, the lower page forms page 7 and the upper page forms page13. For the memory cells on even bit lines connected to word line WL3,the lower page forms page 10 and the upper page forms page 14. For thememory cells on odd bit lines connected to word line WL3, the lower pageforms page 11 and the upper page forms page 15. Memory cells areprogrammed in numerical order according to page number, from page 0 topage 15. Finally, each of the architectures having both even and odd bitlines can be implemented with all the even bit lines located physicallytogether in, for example, the left side of the chip, and all of the oddbit lines located together in, for example, the right side of the chip.

Note that in the embodiments of FIGS. 20A-G, memory cells are programmedalong a NAND string from source side to the drain side. Also, the tablesdepict only an embodiment with four word lines. The various methodsdepicted within the tables can be applied to systems with more or lessthan four word lines. Examples of an architecture using odd/evenprogramming can be found in U.S. Pat. Nos. 6,522,580 and 6,643,188; bothof which are incorporated herein by reference in their entirety. Moreinformation about an architecture that uses all bit line programming canbe found in the following U.S. patent documents incorporated byreference in their entirety: U.S. Patent Application Publication US2004/0057283; U.S. Patent Application Publication US 2004/0060031; U.S.Patent Application Publication US 2004/0057285; U.S. Patent ApplicationPublication US 2004/0057287; U.S. Patent Application Publication US2004/0057318; U.S. Pat. No. 6,771,536; U.S. Pat. No. 6,781,877.

Generally, architectures that program all bit lines together will readdata from all bit lines together. Similarly, architectures that programodd and even bit lines separately will generally read odd and even bitlines separately. However, such limitations are not required. Thetechnology described herein for reading data can be used with all bitline programming or odd/even bit line programming.

FIG. 21 is a flow chart describing one embodiment for reading data fromnon-volatile memory cells. The discussion above with respect to thesense modules discusses how data is read from particular bit lines. FIG.21 provides the read process at the system level. At step 800, a readoperation is performed for a particular page in response to a request toread data (step 798). In one embodiment, when data for a page isprogrammed, the system will also create Error Correction Codes (ECCs)and write those ECCs with the page of data. ECC technologies are wellknown in the art. The ECC process used can include any suitable ECCprocess known in the art. When reading data from a page, the ECCs willbe used to determine whether there are any errors in the data (step802). The ECC process can be performed on he controller, the statemachine or elsewhere in the system. If there are no errors in the data,the data is reported to the user at step 804. For example, data will becommunicated to a controller or host via data I/O lines 320. If an erroris found at step 802, it is determined whether the error is correctable(step 806). The error may be due to the floating gate to floating gatecoupling effect or possibly to other physical mechanisms. Various ECCmethods have the ability to correct a predetermined number of errors ina set of data. If the ECC process can correct the data, then the ECCprocess is used to correct that data in step 808 and the data, ascorrected, is reported to the user in step 810. If the data is notcorrectable by the ECC process, a data recovery process is performed instep 820. In some embodiments, an ECC process will be performed afterstep 820. More details about the data recovery process are describedbelow. After the data is recovered, that data is reported at step 822.Note that the process of FIG. 21 can be used with data programmed usingall bit line programming or odd/even bit line programming.

FIG. 22 is a flow chart describing one embodiment of a process forperforming a read operation for a page (see step 800 of FIG. 21). Theprocess of FIG. 22 can be performed for a page that encompasses all bitlines of a block, only odd bit lines of a block, only even bit lines ofa block, or other subsets of bit lines of a block. In step 840, readreference voltage Vra is applied to the appropriate word line associatedwith the page. In step 842, the bit lines associated with the page aresensed to determine whether the addressed memory cells conduct or do notconduct based on the application of Vra to their control gates. Bitlines that conduct indicate that the memory cells were turned on;therefore, the threshold voltages of those memory cells are below Vra(e.g., in state E). In step 844 the result of the sensing for the bitlines is stored in the appropriate latches for those bit lines. In step846, read reference voltage Vrb is applied to the word lines associatedwith the page being read. In step 848, the bit lines are sensed asdescribed above. In step 850, the results are stored in the appropriatelatches for the bit lines. In step 852, read reference voltage Vrc isapplied to the word lines associated with the page. In step 854, the bitlines are sensed to determine which memory cells conduct, as describedabove. In step 856, the results from the sensing step are stored in theappropriate latches for the bit lines. In step 858, the data values foreach bit line are determined. For example, if a memory cell conducts atVra, then the memory cell is in state E. If a memory cell conducts atVrb and Vrc but not at Vra, then the memory cell is in state A. If thememory cell conducts at Vrc but not at Vra and Vrb, then the memory cellis in state B. If the memory cell does not conduct at Vra, Vrb or Vrc,then the memory cell is in state C. In one embodiment, the data valuesare determined by processor 392. In step 860, processor 392 will storethe determined data values in the appropriate latches for each bit line.In other embodiments, sensing the various levels (Vra, Vrb, and Vrc) mayoccur in different orders.

FIG. 23 includes a flow chart describing one embodiment of a process forrecovering data (step 820). Data may include an error due to thefloating gate to floating gate coupling effect. The process of FIG. 23attempts to read the data while compensating for the floating gate tofloating gate coupling effect. The compensation includes looking at theneighboring word line and determining how the past programming of theneighboring word line has created a floating gate to floating gatecoupling effect. For example, when reading data on word line WLn, theprocess will also read the data of word line WLn+1. If the data on wordline WLn+1 has disturbed the data on WLn, then the read process willcompensate for that disturb. In general, the method proposed here usesdifferent offsets (e.g., 0V, 0.1V, 0.2V, 0.3V) to the read referencevoltages as a function of the state of the memory cell on the neighborword line.

The process depicted in FIG. 23 applies to the full sequence programmingdescribed above with respect to FIG. 17 in which two bits of one logicalpage are stored in each cell and will be read and reported out together.If the memory cell on the neighboring word line is in state E, therewill be no floating gate to floating gate coupling effect. If the memorycell on the neighboring word line is in state A, there will be a smallcoupling effect. If the memory cell on the neighboring word line is instate B, there will be a medium floating gate to floating gate couplingeffect. If the memory cell on the neighboring word line is in state C,there will be a larger floating gate to floating gate coupling effect.The exact coupling effect due to the neighboring word line varies byarray implementation and can be determined by characterizing the device.In one embodiment, the floating gate to floating gate coupling effectdue to a neighbor cell being in state A is an apparent 0.1 volt shift inthreshold voltage. The floating gate to floating gate coupling effectdue to a neighbor memory cell in state B is an apparent 0.2 volt shiftin threshold voltage. The floating gate to floating gate coupling effectdue to a neighboring memory cell in state C is an apparent 0.3 voltshift in threshold voltage. The technology described herein is notlimited to any one set of values for this effect and will vary based onthe implementation.

Step 870 in FIG. 23 includes performing a read operation for theneighboring word line WLn+1. This includes performing the process ofFIG. 22 for the neighboring word line. For example, if a page in wordline WL1 is being read, then step 870 includes performing the process ofFIG. 22 on word line WL2. The results of step 870 are stored in theappropriate latches in step 872. In some embodiments, the read operationperformed for WLn+1 results in determining the actual data stored onWLn+1. In other embodiments, the read operation performed for WLn+1results in a determination of charge levels on WLn+1, which may or maynot accurately reflect the data stored on WLn+1. In step 874, a readoperation is performed for the word line of interest WLn at the normalread points. This includes performing the process of FIG. 22 using Vra,Vrb and Vrc. In some embodiments, the levels and/or the number of levelsused to read WLn+1 may not be exactly the same as those used initiallyto read WLn and simply some approximation of the floating gate thresholdvalue is sufficient for WLn correction purposes. The results of step 874are stored in the appropriate latches for bit lines with memory cellswhere neighbor cell WLn+1 was determined (in step 870) to be in state E.For other bit lines, the data will be disregarded. In step 878, a readoperation will be performed for the word line of interest using a firstset of offsets for the read points. That is, the process of FIG. 22 willbe performed; however, rather than using Vra, Vrb and Vrc, the systemwill use Vra+0.1V, Vrb+0.1V and Vrc+0.1V. In step 888, the results ofstep 878 will be stored for bit lines with memory cells havingneighboring memory cells (e.g., WLn+1) in state A. Data for other bitlines will be disregarded. In step 882, a read operation will beperformed for the word line of interest using a second offset. Theprocess of FIG. 22 will be performed; however, the read reference pointswill be Vra+0.2V, Vrb+0.2V and Vrc+0.2V. In step 884, the results ofstep 882 will be stored in latches for those bit lines with memory cellshaving neighbors (e.g., WLn+1) in state B. In step 886, the readoperation will be performed for the word line of interest using a thirdoffset. Thus, the process of FIG. 22 will use Vra+0.3V, Vrb+0.3V andVrc+0.3V as the read compare points. At step 880, the results of step886 will be stored for those bit lines having memory cells whoseneighboring cells (e.g., WLn+1) are in state C.

In the discussion above, the process of FIG. 23 is performed as part ofthe data recovery step 820 of FIG. 21. In another embodiment, theprocess of FIG. 23 can be used as the initial read process that isperformed in response to a request to read data.

FIG. 24 is a flow chart indicating that the data recovery process (themethod of FIG. 23) can be performed for all the word lines of a blockexcept for the last word line to be programmed. For example, if thereare X+1 word lines, the recovery process can be used for word lines WL0through WLx−1. It would not be necessary to perform the recovery processfor word line WLx (e.g., the word line closest to the drain) becausethat word line has no neighbor that was programmed after it that wouldcause the floating gate to floating gate coupling effect. Although FIG.24 shows an embodiment with a recovery process performed for all theword lines sequentially, in one embodiment described above with respectto FIG. 21, the recovery process can be performed for the word lines atseparate times and only if there were ECC errors that were notcorrectable.

The above-described methods of FIGS. 22 and 23 were discussed withrespect to the full sequence programming storing two bits of one logicalpage of FIG. 17. These processes can be slightly modified when readingdata that was programmed according to the two-step process of FIG. 18storing one bit from each of two logical pages. For example, whenperforming the standard read operation (step 800 of FIG. 21), readingthe lower page would require applying Vra and Vrc to the control gatesof the memory cells and sensing at those read points to determinewhether the data is in state E/C (data 1) or states A/B (data 0) for thelower page. Thus, FIG. 22 would be modified by performing only steps840, 842, 844 and steps 852-860 for a lower page read. For performing aread of the upper page, read compare point Vrb would be used todetermine whether upper page data is for state E/A (data 1) or statesB/C (data 0). Therefore, for an upper page read, the process of FIG. 22would be amended to perform only steps 846, 848, 850, 858 and 860.Additionally, when recovering data (step 820), the process would performthe method of FIG. 25 for recovering data for a lower page and theprocess of FIG. 26 to recover data for an upper page.

In step 930 of FIG. 25, a read operation is performed for theneighboring word line WLn+1 according to the method of FIG. 22. In someembodiments, the read operation performed for WLn+1 results indetermining the actual data stored on WLn+1. In other embodiments, theread operation performed for WLn+1 results in a determination of chargelevels on WLn+1, which may or may not accurately reflect the data storedon WLn+1. The results of that read operation are stored in theappropriate latches in step 932. In step 934, read reference voltage Vrais applied to the word line. In step 936, the data for the bit lines aresensed. In step 938, the results are stored in the appropriate latches.In step 940, read reference voltage Vrc is applied to the word line. Instep 942, data is sensed as discussed above. In step 944, the results ofthe sense step 942 will be stored for bit lines associated with aneighboring cell storing data in state E. In step 946, Vrc plus a firstoffset (e.g., 0.1 volts or another suitable value) will be applied tothe word line for the page being read. In step 948, the data will besensed as discussed above. In step 950, the results of step 948 will bestored for bit lines associated with neighboring cells storing data instate A. Data for other bit lines will be discarded. In step 952, Vrcplus a second offset (e.g., 0.2 volts or other suitable value) will beapplied to the word line associated with the page being read. In step954, data will be sensed using the sense modules, as described above. Instep 956, the results of step 954 will be stored for bit linesassociated with neighboring cells storing data in state B. In step 958,Vrc plus a third offset (0.3 volts or other suitable value) will beapplied to the word lines associated with the page being read. In step960, the sense module will be used to sense the data as described above.In step 962, the results of step 960 will be stored for those bit linesassociated with neighboring cells storing data in state C. In step 964,processor 392 will determine the data values based on the data storedfrom the sensing steps. In step 966, the determined data values fromstep 964 will be stored in latches for eventual communication to theuser requesting the read data. In another embodiment, steps 934-938associated with state A could be performed between steps 962 and 964.

Note that in the process described by FIG. 25, offsets are only appliedto Vrc to separate state B from state C. It is implicitly assumed thatoffsets are not needed when reading at Vra because the usually negativethreshold of the erase state, though affected by WLn+1, is separatedsufficiently far from state A as to not need correction. While this is apractical assumption for current generation memories, it may not be truein future generation memories, and the offset processes described withrespect to Vrc in steps 946-962 may be added to Vra before step 940.

When determining the data values in step 964, if a memory cell conductsin response to Vra, the lower page data is “1.” If the memory cell doesnot conduct in response to Vra and does not conduct in response to Vrc(or Vrc plus the appropriate offset), then the lower page data is also“1.” If the memory cell does not conduct in response to Vra, but doesconduct in response to Vrc (or Vrc plus the appropriate offset), thenthe lower page data is “0.”

The process of FIG. 26 is used to read or recover data for the upperpage. In step 1000, a read operation is performed for the neighboringword line WLn+1 using the method of FIG. 22. In some embodiments, theread operation performed for WLn+1 results in determining the actualdata stored on WLn+1. In other embodiments, the read operation performedfor WLn+1 results in a determination of charge levels on WLn+1, whichmay or may not accurately reflect the data stored on WLn+1. In step1002, the results of step 1000 are stored in the appropriate latches foreach of the bit lines. In step 1004, read reference voltage Vrb isapplied to the word line associated with the page being read. In step1006, sense modules are used to sense the data as described above. Instep 1008, the results of step 1006 are stored for those bit linesassociated with neighboring memory cells storing data in state E. Instep 1010, Vrb plus a first offset (e.g., 0.1V or some other suitablevalue) is applied to the word line. In step 1012, data is sensed, asdescribed above. In step 1014, results from step 1012 are stored forthose bit lines associated with neighboring cells storing data in stateA. In step 1016, Vrb plus a second offset (e.g., 0.2V or anothersuitable value) is applied to the word line associated with the pagebeing read. In step 1018, data is sensed as described above. In step1020, results from step 1018 are stored for those bit lines associatedwith a neighboring cell in state B. In step 1022, Vrb plus a thirdoffset (0.3V or another suitable value) is applied to the word lineassociated with the page being read. In step 1024, data is sensed asdescribed above. In step 1026, results of step 1024 are stored for thosebit lines associated with neighboring memory cells storing data in stateC. In step 1028, processor 392 determines the data values based on thestored sensed data. If a memory cell turned on in response to Vrb (orVrb plus the appropriate offset), then the upper page data is “1.” If amemory cell does not turn on in response to Vrb (or Vrb plus theappropriate offset), then the upper page data is “0.” In step 1030, thedata values determined by processor 392 are stored in the data latchesfor communication to the user.

In another embodiment, rather than using the methods of FIGS. 25 and 26to recover data, the methods of FIGS. 25 and 26 can be used for theinitial data reads performed in response to a request to read data.

FIGS. 25 and 26 are for reading data that are programmed using the upperpage and lower page process of FIG. 18. These two methods of FIGS. 25and 26 can be used to read data programmed by all bit line programmingor odd/even bit line programming. When used with all bit lineprogramming, all bit lines are read simultaneously. When used withodd/even bit line programming, even bit lines are read simultaneously ata first time and odd bit lines are read simultaneously at a differenttime.

FIGS. 27-36 describe processes used to read data that is programmedaccording to the method associated with FIGS. 19A-C. The process of FIG.27 can be implemented as an overall process for reading data that isperformed in response to a read request for a particular one or morepages (or other grouping) of data prior to, separate from and/or inconjunction with using ECCs. In other embodiments, the process of FIG.27 can be performed as part of data recovery step 820 of FIG. 21. Whenreading data as programmed according to the process of FIG. 19, anyperturbation from floating gate to floating gate coupling due toprogramming the lower page of neighboring cells is corrected whenprogramming the upper page of the cell under question. Therefore, whenattempting to compensate for floating gate to floating gate couplingeffect from neighboring cells, the process need only consider thecoupling effect due to the programming of the upper page of neighboringcells. Thus, in step 1060 of FIG. 27, the process reads upper page datafor the neighboring word line. If the upper page of the neighboring wordline was not programmed (step 1062), then the page under considerationcan be read without compensating for the floating gate to floating gatecoupling effect (step 1064). If the upper page of the neighboring wordline was programmed (step 1062), then the page under considerationshould be read using some compensation for the floating gate to floatinggate coupling effect in step 1066. In some embodiments, the readoperation performed for neighboring word line results in a determinationof charge levels on the neighboring word line, which may or may notaccurately reflect the data stored thereon.

In one embodiment, a memory array implementing the programming processof FIG. 19 will reserve a set of memory cells to store one or moreflags. For example, one column of memory cells can be used to storeflags indicating whether the lower page of the respective rows of memorycells has been programmed and another column of memory cells can be usedto store flags indicating whether the upper page for the respective rowsof memory cells has been programmed. In some embodiments, redundantcells can be used to store copies of the flag. By checking theappropriate flag, it can be determined whether the upper page for theneighboring word line has been programmed. More details about such aflag and the process for programming can be found in U.S. Pat. No.6,657,891, Shibata et al., “Semiconductor Memory Device For StoringMulti-Valued Data,” incorporated herein by reference in its entirety.

FIG. 28 describes one embodiment of a process for reading the upper pagedata for a neighboring word line (step 1060 of FIG. 27). In step 1100,read reference voltage Vrc is applied to the word line associated withthe page being read. At step 1102, the bit lines are sensed as describedabove. In step 1104, the results of step 1102 are stored in theappropriate latches. Reading first at Vrc is chosen to uniquelydetermine upper page data since lower page data will normally alreadyhave been written in WLn+1 and reading at Vra or Vrb will not guaranteea unique result since distribution 750 (FIG. 19C) may overlap thesevalues.

In step 1106, the system checks the flag indicating upper pageprogramming associated with the page being read. In one embodiment, thememory cell storing the flag will store data in state E if the flag isnot set and in state C if the flag is set. Therefore, when thatparticular memory cell is sensed at step 1102, if the memory cellconducts (turns on), then the memory cell is not storing data in state Cand the flag is not set. If the memory cell does not conduct, then it isassumed in step 1106 that the memory cell is indicating that the upperpage has been programmed.

In another other embodiment, the flag can be stored in a byte. Ratherthan storing all bits in state C, the byte will include a unique 8-bitcode representing the flag and known to the state machine 312, such thatthe 8-bit code has at least one bit in state E, at least one bit instate A, at least one bit in state B and at least one bit in state C. Ifthe upper page has not been programmed, the byte of memory cells willall be in state E. If the upper page has been programmed, then the byteof memory cells will store the code. In one embodiment, step 1106 isperformed by checking whether any of the memory cells of the bytestoring the code do not turn on in response to Vrc. In anotherembodiment, step 1106 includes addressing and reading the byte of memorycells storing the flag and sending the data to the state machine, whichwill verify whether the code stored in the memory cells matches the codeexpected by the state machine. If so, the state machine concludes thatthe upper page has been programmed.

If the flag has not been set (step 1108), then the process of FIG. 28terminates with the conclusion that the upper page has not beenprogrammed. If the flag has been set (step 1108), then it is assumedthat the upper page has been programmed and at step 1120 read voltageVrb is applied to the word line associated with the page being read. Instep 1122, the bit lines are sensed as discussed above. In step 1124,the results of step 1122 are stored in the appropriate latches. In step1126, read reference voltage Vra is applied to the word line associatedwith the page being read. In step 1128, the bit lines are sensed. Instep 1130, the results of step 1128 are stored in the appropriatelatches. In step 1132, processor 392 determines the data value stored byeach of the memory cells being read based on the results of the threesensing steps 1102, 1122 and 1128. At step 1134, the data valuesdetermined in step 1132 are stored in the appropriate data latches foreventual communication to the user. In step 1132, processor 392determines the values of the upper page and lower page data using wellknown simple logic techniques dependent on the specific state codingchosen. For example, for the coding described in FIG. 19, the lower pagedata is Vrb* (the complement of the value stored when reading at Vrb),and the upper page data is Vra* OR (Vrb AND Vrc*). The process of FIG.20, though here described as being used to read WLn+1, can also be usedto read WLn as described below. When used to read WLn+1 as in step 1160of FIG. 27, what is desired is not simply the data but a determinationof the presence of upper page data. This determination is made using themethod described in FIG. 31 below. It should be noted that the valueschosen for Vra, Vrb, and Vrc when reading WLn+1 may differ from thosechosen when reading WLn.

FIG. 29 is a flow chart describing one embodiment of a process forreading data of the word line under consideration when the system doesnot need to compensate for floating gate to floating gate coupling froma neighboring word line (see step 1064 of FIG. 27). In step 1150, it isdetermined whether the read is for the upper page or lower pageassociated with the word line under consideration. If the read is forthe lower page, then in step 1152 read reference voltage Vrb is appliedto the word line associated with the page being read. In step 1154, thebit lines are sensed. In step 1156, the results of sensing step 1154 arestored in the appropriate latches. In step 1158, the flag is checked todetermine if the page contains upper page data. If there is no flag,then any data present will be in the intermediate state and Vrb was theincorrect threshold to use and the process continues at step 1160. Instep 1160, Vra is applied to the word line, the bit lines are re-sensedat step 1162, and in step 1164 the result is stored. In step 1166 (aftereither step 1164, or step 1158 if the flag is set, processor 392determines a data value to be stored. In one embodiment, when readingthe lower page, if the memory cell turns on in response to Vrb (or Vra)being applied to the word line, then the lower page data is “1”;otherwise, the lower page data is “0.”

If it is determined that the page address corresponds to the upper page(step 1150), an upper page read process is performed at step 1170. Inone embodiment, the upper page read process of step 1170 includes thesame method described in FIG. 28, which includes reading the flag andall three states since an unwritten upper page may be addressed forreading, or another reason.

FIG. 30 depicts a flow chart describing one embodiment of a process forreading data while compensating for floating gate to floating gatecoupling effect (see step 1066 of FIG. 27). In step 1200 of FIG. 30, thesystem determines whether to use an offset to compensate for thefloating gate to floating gate coupling. This is performed separatelyfor each bit line. The appropriate processor 392 will determine whichbit lines need to use the offset based on the data from the neighboringword lines. If a neighboring word line is in state E or B (or has chargeapparently indicating state E or B), then the particular word line beingread need not compensate for the floating gate to floating gate couplingeffect. The assumption is that if it is in state E it hasn't contributedto any coupling because the threshold hasn't moved since the currentword line was written. If it is in state B, it got there from B′, andthe movement from B′ to B is small and can be neglected. In oneembodiment, the process of step 1200 can be performed concurrently withstep 1060. For example, FIG. 31 provides a chart explaining steps toperform a determination whether to use an offset for a particular bitline. The first step is to perform a read process using Vra. The secondstep is to perform a read using Vrb. When reading at Vra, a latch storesa 1 if the memory cell is in state E and a 0 if the memory cell is instates A, B, C or. When reading at Vrb, the latch will store a 1 forstates E and A, and store a 0 for states B and C. The third step of FIG.31 includes performing an XOR operation on the inverted results from thesecond step with the results from step 1. In the fourth step, a read isperformed using Vrc at the word line. A latch stores a 1 for states E, Aand B, and stores a 0 for state C. In the fifth step, the results ofstep 4 and step 3 are operated by a logical AND operation. Note thatsteps 1, 2 and 4 may be performed as part of FIG. 28. Steps 3 and 5 ofFIG. 31 can be performed by dedicated hardware or by processor 392. Theresults of step 5 are stored in a latch with 1 being stored if no offsetis needed and 0 being stored if an offset is needed. The offset is usedto compensate for the floating gate to floating gate coupling. Thus, aread offset will be required for those cells that are read on WLn thathave neighboring memory cells on WLn+1 that are in the A or C state.This approach requires only one latch to determine whether to correctWLn or not, in contrast to the previous methods that store the full datafrom WLn+1, requiring two or more latches.

Looking back at step 1202 of FIG. 30, it is determined whether the pagebeing read is the upper page or lower page. If the page being read isthe lower page, then Vrb is applied to the word line associated with thepage being read (step 1204). Note that for the binary state codingdescribed in FIG. 19, reading at Vrb is sufficient to determine thelower page data, in contrast to the binary state coding described inFIGS. 17 & 18 in which reading at Vrb is used to determine the upperpage data. In step 1206, the bit lines are sensed. In step 1208, theresults of step 1206 are stored in the appropriate latches associatedwith the bit lines. In step 1210, Vrb plus an offset is applied to theword line being read. In step 1212, the bit lines are sensed. In step1214, the results of the sensing of step 1212 are used to overwrite theresults stored in step 1208 for the bit line for which it was determinedat step 1200 to use an offset. If the particular bit line is determinednot to have to use an offset, then the data from step 1212 is notstored. In step 1216, processor 392 will determine whether the data is 1or 0 for the lower page. If the memory cell turned on in response to Vrb(or Vrb plus the offset, if appropriate), then the lower page data is 1;otherwise, the lower page data is 0. At step 1218, the lower page datais stored in the appropriate latches for communication to the user.

If it is determined at step 1202 that the page being read is the upperpage, then the upper page correction process is performed at step 1220.FIG. 32 provides a flow chart describing the upper page correctionprocess. In step 1250 of FIG. 32, read reference voltage Vrc is appliedto the word line associated with the page being read. In step 1252, thebit lines are sensed. In step 1254, the results of the sensing step arestored in the appropriate latches. In step 1256, Vrc plus an offset(e.g., 0.1V) is applied to the word line associated with the page beingread. In step 1258, the bit lines are sensed. In step 1260, the resultsof the sensing step 1258 are used to overwrite the results stored instep 1254 for any bit line for which the offset is required (see step1200). At step 1270, Vrb is applied to the word line. In step 1272, thebit lines are sensed. In step 1274, the results of sensing step 1272 arestored. In step 1276, Vrb plus the offset is applied to the word lineassociated with the page being read. In step 1278, the bit lines aresensed. In step 1280, the results of step 1278 are used to overwrite theresults stored at step 1274 for those bit lines for which the offset isrequired (see step 1200). In step 1282, Vra is applied to the word lineassociated with the page being read. In step 1284, the bit lines aresensed. In step 1286, the results of the sensing step 1284 are stored inthe appropriate latches. In step 1288, Vra plus the offset is applied tothe word line associated with the page being read. Note that the offsetused in steps 1288, 1280 and 1256 are the same, implicitly assuming thatthe coupling to WLn from programming the upper page of WLn+1 in movingfrom state E to state A is approximately the same as when moving fromstate B′ to C. In other embodiments, the offsets can differ. In step1290, the bit lines are sensed. In step 1292, the results of step 1290are used to overwrite the results stored in step 1286 for those bitlines for which the offset is required (see step 1200). In someembodiments, the margin between state E and state A is sufficient thatthe offset associated with Vra is not necessary and steps 1288 through1292 may be skipped. In step 1294, the processor 392 determines the datavalues in the same manner as previously described with respect to FIG.28 or another method known in the art. In step 1296, the data valuesdetermined by the processor 392 are stored in the appropriate datalatches for communication to the user. In other embodiments the order ofreading (Vrc Vrb, Vra) may be changed.

In the above discussion with respect to FIG. 27, an example is discussedinvolving the reading of a page of data. It is likely, but not required,that a request to read data will require the reading of multiple pagesof data. In one embodiment, to speed up the process of reading multiplepages of data, the read process will be pipelined such that the statemachine will execute a next page sensing while the user is transferringout the previous page of data. In such an implementation, the flag fetchprocess (see e.g., step 1006 of FIG. 28) may interrupt the pipelinedread process. To avoid such an interruption, one embodiment contemplatesreading the flag for a given page when that page is read and using thewired-OR detection process mentioned above to check the flag (ratherthan reading the flag and sending it to the state machine). For example,during step 1060 of FIG. 27 (reading the neighboring word line), theprocess first reads data using Vrc as the reference voltage. At thatpoint, if the wired-OR line indicates that each state stores data 1,then the upper page has not been programmed; therefore, no compensationis needed and the system will read without compensating for the floatinggate to floating gate coupling (step 1064). If the flag is a one-bytecode that includes data in each data state, at least the flag memorycells would have data in state C if the flag is set. If the wired-ORline indicates that no memory cells have data in state C, then the statemachine concludes that the flag has not been set; therefore, the upperpage for the neighboring word line has not been programmed andcompensation for floating gate coupling is not needed.

FIG. 32A is a timing diagram that explains one embodiment for performingpipelined reads as discussed above. FIG. 32A depicts two signals. Signal1300 represents a Ready/Busy signal communicated from the memory systemto the controller (or host/user) that when low indicates that the memorysystem is not yet ready to send data on I/O lines 320 and when highindicates that data is available to be transferred. FIG. 32A shows apipelined read process in response to a read request that pertains tolower and upper pages for memory cells along word lines WLn, WLn+1,WLn+2, . . . Signal 1300 first includes period 1300A, which correspondsto waiting for the first set of data to be ready for communication tothe user. During period 1300B, data stored in the lower page of memorycells connected to word line WLn is transferred out to the user via I/Olines 320. During period 1300C, data stored in the upper page of memorycells connected to word line WLn is transferred out to the user via I/Olines 320. During the next period, data stored in the lower page ofmemory cells connected to word line WLn+1 is transferred out, and so on.

Signal 1302 of FIG. 32A is a symbolic representation of what ishappening within the memory system. The first data to be read is thelower page of memory cells connected to word line VvLn. During period1302A, the neighboring word line WLn+1 is read (e.g., three readoperations—at Vra, Vrb and Vrc). In period 1302B, the flag is fetchedand communicated to the state machine to determine if the upper page wasprogrammed. Note that because the pipeline has not started, the statemachine can perform the read fetch. Alternately the wired-OR processafter reading at Vrc (that was discussed above) can be employed to checkthe flag. In step 1302C, the lower page of WLn is read, usingcompensation as necessary as determined by the flag status. Duringperiod 1302D WLn lower page data is placed in the output registers.

In period 1302E, the neighboring word line WLn+1 is read (e.g., threeread operations—at Vra, Vrb and Vrc). This step may not be needed sinceit was already done before. However, since the time associated with1302E, 1302F, and 1302G is typically less than that associated with step1300B, it may be performed for consistency of operation. Furthermore, insome embodiments there may not be enough latches available tosimultaneously save the data associated with both WLn and WLn+1. Duringperiod 1302F, the flag for the WLn+1 is read by performing the wired-ORprocess after reading at Vrc, thereby avoiding the full flag fetch thatstops the pipeline. During period 1302G, the upper page of WLn is read,using compensation as necessary. During period 1302H, signal 1302 dropswhile the WLn upper page data is transferred from the internal latchesto the output registers.

After reading the lower and upper pages for WLn, the system will thenread the lower and upper pages for WLn+1, and so on until all of theread data is provided to the user. In period 1302I, the new neighboringword line WLn+2 is read (e.g., three read operations—at Vra, Vrb andVrc). During period 1302J, the flag for the WLn+2 is read by performingthe wired-OR process after reading at Vrc, thereby avoiding the fullflag fetch that stops the pipeline. During period 1302K, the lower pageof WLn+1 is read, using compensation as necessary. The process willcontinue, as discussed above. In some embodiments it may be useful toreduce the number of reads by reading WLn+1 data once followed byreading WLn upper and lower page data simultaneously and saving theupper page data rather than re-reading it later. One advantage ofreducing the number of read steps can be to minimize power consumption.

The above embodiments use multiple reads at different reference points.While this provides an accurate means for compensating for the floatinggate to floating gate coupling, it adds extra time to the read process.Another embodiment is to modify the sense amplifier to add a correctionfeature. If the sense amplifier can be modified to sense a differenttrip point depending on the neighboring word line data, then a singlesensing operation will give the final corrected data. This method can bereliable and save time. The drawback is the increase in sense amplifierlayout area.

Looking back at FIG. 8, the sensing trip point is set based on capacitorCSA 652 of sense amplifier 600. Different trip points can be set for thesame sensing process by using a different capacitor on the SEN modedepending on data from a neighboring word line. When no correction isneeded, use a large capacitor. When correction is needed, use a smallercapacitor. FIG. 33 depicts a sense module 380′ that is similar to thesense module 380 of FIG. 8. However, sense amplifier 600′ includes anadditional capacitor 1382 that is connected or not connected based ondata from the neighboring word line. Capacitor 1382 is connected to theSEN mode via transistor 1380. Transistor 1380 is connected to thereadout bus 532 via transistor 1384 and gate 530. Data from theneighboring word line is provided via readout bus 532 and transistor1384 to the node C for transistor 1380. If data 1 is provided at node C,then no correction is needed due to the floating gate to floating gatecoupling from a neighbor memory cell. If data 0 is provided at node C,then correction is needed. If no correction is needed, capacitor 1382 iselectrically connected to SEN node. If correction is needed, capacitor1382 is disconnected from SEN node. When the extra capacitor is notattached to SEN node, a smaller cell current associated with the higher(disturbed) threshold will charge the voltage on the capacitorequivalently to the lower threshold (undisturbed) state charging ahigher capacitance.

Although FIG. 33 depicts selectively adding an additional capacitor,other embodiments can selectively add more capacitors to achievedifferent compensation effects. Additionally, some embodiments may haveall of the capacitors connected to the SEN node selectively coupled sothat each possibility (e.g., no compensation, compensation 1,compensation 2, . . . ) connects a different set of capacitors. In someimplementations, other capacitive devices can also be used.

FIG. 34 provides a flow chart describing one embodiment for using thetwo capacitors of FIG. 33. The method of FIG. 34 provides an alternativeembodiment to the method of FIG. 30. In step 1320 of FIG. 34, adetermination is made on a bit line basis whether to use the offset forthat particular bit line. This is similar to step 1200 of FIG. 30. Instep 1322, it is determined whether the read process is for the lowerpage or the upper page. If the read process is for the lower page, themethod continues at step 1324. If the offset is required, then theadditional capacitor is disconnected from the SEN 2 node. If the offsetis not required, then the additional capacitor remains connected to theSEN 2 node. In step 1326, Vrb is applied to the word line associatedwith the page being read. In step 1328, the bit lines are sensed. Instep 1330, the results of the sensing step 1328 are stored. In step1332, processor 392 determines the data value stored. In one embodiment,when reading the lower page, if the memory cell turns on in response toVrb being applied to the word line, then the lower page data is “1”;otherwise, the lower page data is “0.” In step 1334, the data valuesdetermined by the processor are stored in the appropriate latches, forread out by the user.

If it is determined that the read process is for the upper page (step1322), then the process continues to step 1340. If step 1320 concludedthat the offset is required, then the additional capacitor isdisconnected (step 1340). If the offset is not required, then theadditional capacitor remains connected to the SEN 2 node. In step 1342,an upper page read process is performed, as described above with respectto FIG. 28.

As described above, when a large number of memory cells are sensed inparallel, their combined currents can result in significant voltage dropand a ground loop with finite resistance. This results in a source linebias. One embodiment utilizes this source line bias to compensate forthe floating gate to floating gate coupling effect. One embodimentdescribed above that takes into account the source line bias includesmultiple sensing steps (also called strobes). During the first strobe,all appropriate bit lines will be connected to the source line. Duringthe second strobe, a smaller subset of bit lines will be connected tothe source line. The first strobe is performed with a higher source lineoffset voltage, which will indicate that the conducting cells areactually less conducting than they would be with no source line voltageoffset. This is equivalent to a threshold voltage shift to a highervalue. The process proposed is that the first strobe unconditionallysaves data in the data latch. At the second strobe, check if some cellson the word line being read require compensation due to the floatinggate coupling effect with the neighboring word line. If so, overwritethe data from the first strobe with the data from the second strobe forthe cells that require correction. For cells that do not needcorrection, do not overwrite the data and discard the data from thesecond strobe. One advantage of using this method is that it reduces theread time as the data correction is hidden within the normal readroutine.

A disadvantage of the source line bias method is that the value of theoffset depends on the data pattern. If there are more conducting cellsat a certain sensing level, the source voltage will be larger and morecorrection will be done. If there are less conducting cells at a certainsensing level, the source voltage will be less and fewer correction willbe done. Assuming every page has perfectly random data, the shift wouldbe a nearly constant value for every page. With two bits per memorycell, 25% of the bit lines may be in state E, 25% in state A, 25% instate B and 25% in state C. Reading at Vra, there will be 25% of the bitlines conducting. Reading at Vrc, there will be 75% of the bit linesconducting, resulting in more correction when reading state C than whenreading state A.

FIG. 35 provides a flow chart describing one embodiment for reading withcompensation (see step 1066), based on using the two strobe cyclesdiscussed above. In step 1400, the system determines whether to choosethe offset for the particular bit lines. This is similar to step 1200,described above. If the read process is for the lower page (step 1402),then the process continues at step 1404. If the read process is for theupper page, then the process continues at step 1424. In step 1404, thereference voltage Vrb is applied to the word line associated with thepage being read. At step 1406, the bit lines are sensed. At step 1408,results of the sensing step 1406 are stored in the appropriate latches.Steps 1404-1408 are the first strobe. In step 1410, those bit linesdetermined to have a high current during the first strobe are turned offfor the second strobe. In step 1412, the second strobe is started byapplying Vrb to the word line. In step 1414, the bit lines are sensed.In step 1416, those bit lines for which the correction is required willhave the data from step 1414 used to overwrite the data from step 1408.In step 1418, processor 392 determines the data values stored. In oneembodiment, when reading the lower page, if the memory cell turns on inresponse to Vrb being applied to the word line, then the lower page datais “1”; otherwise, the lower page data is “0.” In step 1420, thedetermined data values are stored in the appropriate latches, forcommunication to the user.

FIG. 36 provides a flow chart describing one embodiment of a process forperforming upper page read with correction based on the source current(step 1424 of FIG. 35). In step 1502, reference voltage Vrc is appliedto the word line associated with the page being read. In step 1504, thebit lines are sensed. In step 1506, the results are stored in theappropriate latches. In step 1508, those bit lines with high current areturned off for the second strobe. The first strobe includes steps1502-1506. In step 1510, the second strobe is started by applying Vrc tothe same word line. In step 1512, the bit lines are sensed. In step1514, the results stored in step 1506 are overwritten by the results ofstep 1512 for those bit lines for which correction is required. In step1516, the system checks the flag for the upper page programming, asdescribed above. If the flag is set indicating that there is dataprogrammed in the upper page, the process continues to step 1522. If theflag was not set, then at step 1520 the process of FIG. 36 isterminated, concluding that the upper page has not been programmed.Other processes or timing for checking the flag discussed above can alsobe used.

In step 1522, the first strobe is performed by applying Vrb to the wordline associated with the page being read. In step 1524, the bit linesare sensed. In step 1526, the results are stored in the appropriatelatches. In step 1528, those bit lines with high current in the firststrobe are turned off for the second strobe. In step 1540, the secondstrobe is commenced by applying Vrb to the word lines. In step 1542, thebit lines are sensed. In step 1544, the results from step 1526 areoverwritten by the results from step 1542 for those bit lines for whichthe offset is required. In step 1546, the first strobe is performed byapplying Vra to the word line associated with the page being read. Instep 1548, the bit lines are sensed. In step 1550, the results arestored from step 1548. In step 1552, the bit lines with high currentduring the first strobe are turned off for the second strobe. In step1554, the second strobe is commenced by applying Vra to the word line.In step 1556, the bit lines are sensed. In step 1558, the results storedin step 1550 are overwritten by the results from step 1556 for those bitlines for which offset is required. In some embodiments, the marginbetween state E and state A is sufficient that the offset associatedwith Vra is not necessary and steps 1552 through 1558 may be skipped. Instep 1560, processor 392 determines the data values stored based on theresults stored in the latches. This is performed as discussed above withrespect to step 1132. The data values determined by processor 392 arestored in the appropriate latches in step 1562.

As a result of the ability to reverse the effects of the floating gateto floating gate coupling, the margins between threshold voltagedistributions can be made smaller or the memory system can programfaster.

In the above embodiments, there were three additional reads for theneighboring word lines to read data stored in the four states. In otherembodiments, fewer than three reads can be performed, thereby reducingthe number of different offsets used. This will result in reducingresolution of the offsets. Additionally, more than three reads can beused to have a finer adjustment of the offset. In some embodiments,portions of the processes described above can be performed off chip.

The foregoing detailed description of the invention has been presentedfor purposes of illustration and description. It is not intended to beexhaustive or to limit the invention to the precise form disclosed. Manymodifications and variations are possible in light of the aboveteaching. The described embodiments were chosen in order to best explainthe principles of the invention and its practical application, tothereby enable others skilled in the art to best utilize the inventionin various embodiments and with various modifications as are suited tothe particular use contemplated. It is intended that the scope of theinvention be defined by the claims appended hereto.

1. A flash memory system, comprising: a set of flash memory elementsarranged in columns and rows using a set of bit lines and word lines,each flash memory element in a common column is connected to a commonbit line, each flash memory element in a common row is connected to acommon word line; one or more write control circuits in communicationwith said set of flash memory elements, said one or more write controlcircuits simultaneously program flash memory elements connected to aparticular word line on all of said set of bit lines; and read circuitsin communication with said bit lines to determine a quantity related toone of two or more adjacent flash memory elements along respective bitlines, said read circuits determine an offset for each flash memoryelement connected to said particular word line from a predetermined setof offsets based on a charge level stored in an adjacent flash memoryelement, said read circuits perform a set of read processes for saidflash memory element connected to said particular word line, each readprocess uses a different one of said predetermined set of offsets, eachof at least a subset of said flash memory elements connected to saidparticular word line provides final data from an appropriate one of saidread processes associated with the offset determined for the respectiveflash memory element.
 2. A flash memory system according to claim 1,wherein: said set of flash memory elements comprise a block; and saidflash memory element connected to said particular word line store atleast a first page of data.
 3. A flash memory system according to claim1, wherein: there is at least one read process for each offset.
 4. Aflash memory system according to claim 1, wherein: set of flash memoryelements are NAND flash memory devices.
 5. A flash memory systemaccording to claim 1, wherein: said set of flash memory elements aremulti-state flash memory devices.